AN ABSTRACT OF THE THESIS OF
Rachel E. Sours-Page for the degree of Doctor of Philosophy in Geology presented on
July 17, 2000. Title: Magmatic Processes at Mid-Ocean Ridges: Evidence from Lavas
and Melt Inclusions from the Southeast Indian Ridge, the Endeavour Segment of the Juan
de Fuca Ridge, and the northern East Pacific Rise
Abstract approved:
Signature redacted for privacy.
Roger L. Nielsen
Magmatic processes control the chemical compositions of all lavas erupted at
mid-ocean ridges. In this thesis, I present studies of magmatic processes on three
different mid-ocean ridges to determine which processes are in action and to what extent
each has affected the chemistry of mid-ocean ridge basalts at each location.
On the Endeavour Segment, Juan de Fuca Ridge, major and trace element data
from enriched and depleted lavas and melt inclusions indicate that lavas and melt
inclusions are the results of partial melting of a heterogeneous source. Trace element
models suggest that depleted lavas are formed from variable degrees of partial melting of
a refractory harzburgite source, while enriched lavas result from very small degrees of
melting of a clinopyroxenite source.
Major and trace element data from axial and seamount lavas and melt inclusions
from the northern East Pacific Rise indicate that chemical differences between axial and
seamount magmas result from varying exposure to crustal and axial magma chamber
processes. Seamount lavas and inclusions are more crystal rich and contain a greater
number of inclusions that are generally more primitive and exhibit a larger compositional
range in both the incompatible and trace elements. Seamount lavas leave the axial
magma chamber before axial lavas, and thereby miss the further fractionation and crystal
sorting.
Major element data from Southeast Indian Ridge lavas suggest that the dominant
control of MORB chemistry is mantle temperature. Lavas from this region range from
high Na8, low Fe8 in the east to low Na8, high Fe8 in the west, suggestive of higher
high Na8, low Fe8 in the east to low Na8, high Fe8 in the west, suggestive of higher
pressures and extents of melting in the western part of the study area. Variable degrees
and pressures of melting are consistent with a mantle temperature gradient which
extends from hot mantle below the Amsterdam-St Paul hotspot to cold mantle below the
Australian-Antarctic Discordance.
©Copyright by Rachel E. Sours-Page
July 17, 2000
All Right Reserved
Portions of this manuscript have appeared in Contributions to Mineralogy and Petrology
Magmatic Processes at Mid-Ocean Ridges: Evidence from Lavas and Melt Inclusions
from the Southeast Indian Ridge, the Endeavour Segment of the Juan de Fuca Ridge,
and the Northern East Pacific Rise
By
Rachel E. Sours-Page
A THESIS
submitted to
Oregon State University
in partial fulfillment of
the requirements for the
degree of
Doctor of Philosophy
Presented July 17, 2000
Commencement June 2001
Doctor of Philosophy thesis of Rachel E. Sours-Pag presented July 17, 2000.
APPROVED:
Signature redacted for privacy.
Major Professor, representinGeology
Signature redacted for privacy.
Chair of Department
1 Geo
sciences
Signature redacted for privacy.
Dean of Grat'Schoo1
I understand that my thesis will become part of the permanent collection of the Oregon
State University libraries. My signature below authorizes release of my thesis to any
reader upon request.
Signature redacted for privacy.
Rachel E. Sours-Page, Author
ACKNOWLEDGEMENTS
I would like to thank Roger Nielsen for his never-ending academic and financial
support throughout my graduate school career. I also thank Anita Grunder, Dave
Graham and Dave Christie for their advice and criticism, when needed. I would like to
acknowledge my Graduate Council Representative, Barbara Gartner, and thank her for
her time. Finally, I would like to thank the entire geology group for making my time
here enjoyable. In particular, I am indebted to Mark Hilyard, Melanie Kelman, Heather
Petcovic, Mike Winkler, Martin Hannigan, Ed Kohut, and Joe Licciardi for keeping me
sane through the tough times and Christopher Boyette for showing me that there are
other things in life. This research was supported in part by National Science
Foundation grant numbers OCE-9503782 and OCE-9730079.
CONTRIBUTION OF AUTHORS
Dr. Roger Nielsen was involved in the design, data analysis, interpretation, and
writing of each manuscript. For the second chapter, Dr. Jill Karsten provided the
samples and Dr. Kevin Johnson assisted in data collection and modeling, and both were
involved in the revision process. Dr. Rodey Batiza assisted in the design and revision
of the third chapter. For the fourth chapter, Dr. David Christie provided both the
samples and the data reported therein. Drs. David Christie, David Graham and Laura
Magde assisted in the design, data interpretation and revision of the fourth chapter.
TABLE OF CONTENTS
Page
Introduction
1
Local and Regional Variation of MORB Parent Magmas: Evidence from Melt
Inclusions from the Endeavour Segment of the Juan de Fuca Ridge
5
Abstract
6
Introduction
7
The Phenomenon of Over-Enrichment
The Use of Mg#
9
10
Geologic Setting and Samples
11
Experimental and Analytical Procedure
19
Rehomogenization Technique
Electron Microprobe
Ion Microprobe
Results
Major Elements
Minor and Trace Elements
The Relationship Between Melt Inclusions and Their Host Lavas
Phase Equilibria Modeling
Melting Models
19
19
20
25
25
25
35
38
39
Discussion
42
Conclusions
49
Acknowledgments
50
Parental Magma Diversity on a Fast-Spreading Ridge: Evidence From Olivine
and Plagioclase-Hosted Melt Inclusions in Axial and Seamount Lavas From
the Northern East Pacific Rise
Abstract
51
52
TABLE OF CONTENTS (continued)
Page
Introduction
53
Geologic Setting
55
Axial Volcanism
Seamount Volcanism
Sample Information
Experimental and Analytical Methods
Rehomogenization Technique
Effects of Over and Under-Heating During Rehomogenization
Electron Microprobe
Ion Microprobe
Results
Mineralogy
Melt Inclusion Major and Trace Element Behavior
Relationship Between Melt Inclusions and Their Crystal Hosts
Discussion
Global Framework
Differing Histories of Plagioclase and Olivine
Magmatic Processes
55
59
59
60
60
63
63
64
65
65
65
75
81
81
83
83
Conclusions
90
Acknowledgements
93
Linking the Local, Regional and Global Petrologic Systematics of Mid-Ocean
Ridges: The Southeast Indian Ridge Case
94
Abstract
95
Introduction
96
Geologic Setting
97
TABLE OF CONTENTS (continued)
Page
How Large is the Mantle Temperature Gradient?
100
Petrologic Expectations
101
Methods
101
Sample Information
Na8 Calculation
115
115
Results
116
Discussion
127
Crystallization Models
Similarities and Differences Between the SEIR and the Global Array
127
128
Conclusions
137
Acknowledgements
138
Summary
139
Bibliography
141
LIST OF FIGURES
Figure
Page
Bathymetric map for the Endeavour Segment, including sample locations
12
Al203 versus Mg# for inclusions from Endeavour Segment samples
E-1, 0-2, E-5, and E-32
12
Representative minor element electron microprobe analyses of inclusions
from Endeavour Segment lavas
12
Comparison of electron microprobe (EMP) and ion microprobe analyses
of Ti in melt inclusions
21
Electron and ion microprobe trace element analyses of inclusions from the
Endeavour Segment lavas
27
Ion microprobe analyses of inclusions from the Endeavour Segment lavas
27
Chondrite-normalized REE abundances in inclusions from the Endeavour
Segment lavas
27
Histogram representing the frequency of melt inclusion K20 values in each
host lava plotted against the host lava K20 composition
37
Cartoon representing the melting regimes necessary to create B- and
N-MORB lavas
43
Correlation of abyssal peridotite clinopyroxene analyses and calculated
fractional melting liquid line of descent from Johnson et al. (1990)
44
Map view of the northern East Pacific Rise, including sample locations
56
Host crystal and lava compositions versus associated melt inclusion MgO
66
Major and minor elements versus MgO for representative inclusions from
axial and seamount lavas from the East Pacific Rise
66
Electron microprobe analyses of minor and trace elements of melt inclusions
from axial and seamount lavas of the northern East Pacific Rise
76
La and Ba versus K20 and Ti02
76
LIST OF FIGURES (continued)
Figure
Page
Ti/Zr and La/Sm versus MgO and K20
76
Cartoon representing the different trends associated with mixing and
fractionation, depending on the order in which processes take place
85
K20 versus Mg# for plagioclase-hosted melt inclusions in sample E-5 from
the Endeavour Segment of the Juan de Fuca Ridge
85
Cartoon representing the three different host lava-melt inclusion
relationships observed in the N-EPR lavas
85
Cartoon representing the axial plumbing system necessary to supply
magmas to both axial and seamount eruptions
91
Map of the Southeast Indian Ridge between 88° and 118°E
99
Equilibrium crystallization liquid line of descent models for CaO, K20,
Na20 and Ti02 versus MgO
117
Major and minor element variation of the Southeast Indian Ridge lavas
with axial depth and longitude
117
Na8, Fe8, and CaO/Al203 variations with axial depth and longitude,
calculated according to the equations in Klein and Langmuir (1987)
117
Comparison of Southeast Indian Ridge with Global Array in Na8, Fe8, and
CaO/Al203 versus depth
122
Ti02 versus MgO for segments Cl 7 and C 14 of the Southeast Indian Ridge
124
SEIR Na8 versus Fe8 with different symbols for each segment
125
ASP, SEIR, and AAD Na8 versus longitude and depth
131
LIST OF TABLES
Page
Table
Major and trace element analyses of melt inclusions from the Endeavour
Segment lavas of the Juan de Fuca Ridge
22
Modes, partition coefficients, and starting compositions used for trace
element modeling
45
Major and trace element analyses of axial and seamount host lavas
61
Major element analyses of melt inclusions from axial and seamount lavas
of the northern East Pacific Rise
70
Trace element analyses of selected melt inclusions from axial and seamount
lavas of the northern East Pacific Rise
70
Chemical group major element analyses for Southeast Indian Ridge lavas
102
Comparison of linear equations of Na8, Fe8 and CaO/Al203 from previous
research and this paper
129
Predicted and actual values of Na8, Fe8 and CaO/Al203 for the SEIR and
regional datasets
130
Observations and conclusions based on the Southeast Indian Ridge major
element data
136
MAGMATIC PROCESSES AT MID-OCEAN RIDGES: EVIDENCE FROM
LAVAS AND MELT INCLUSIONS FROM TE SOUTHEAST INDIAN RIDGE,
THE ENDEAVOUR SEGMENT OF THE JUAN DE FUCA RIDGE, AND THE
NORTHERN EAST PACIFIC RISE
INTRODUCTION
Rachel Sours-Page
July, 2000, 4 pages
2
The global mid-ocean ridge system is an underwater chain of volcanoes that
extends for more than 60,000 km under all of the earth's major oceans. It is arguably the
simplest and most important magmatic system on Earth, being devoid of the
complications associated with the thicknesses of continental crust or the complexity of
the arc setting, and forming the oceanic crust which covers more than 70% of the Earth's
surface. In addition, mid-ocean ridges provide points of exit for much of the heat that is
produced from radioactive decay within the Earth. The study of mid-ocean ridges leads
to an understanding of one of the key components of the plate tectonic theory.
There are many factors which influence crustal accretion, but they can all be
related back to three inter-related parameters: mantle temperature, mantle composition,
and ridge spreading rate. Lavas formed at mid-ocean ridges, termed mid-ocean ridge
basalts (MORB) undergo many similar processes, regardless of where on Earth they are
created. In the model put forth by Klein and Langmuir (1987), upwelling mantle
undergoes adiabatic decompression, which induces partial melting of the mantle source at
its solidus. The region bounded by the solidi is refered to as the melting regime. Within
this region, the hotter the mantle, the greater the depth of the solidus. The greater the
depth of the solidus, the longer the melting column, which leads to more melt that is
formed, and therefore, a greater thickness of overlying ocean crust accreted. When the
mantle is colder, the solidus is intersected at a shallower depth, leading to less melt
formation and thinner overlying crust.
Although petrologists have a basic understanding of how melt generation works,
in general, there are still many questions, including:
Where, or at what depth, does melting begin?
What is the composition of the source mantle, and how much of it is melting?
How much melt is formed?
How is melt extracted from its source? Is it continuously removed, as in pure
fractional melting, or does it accumulate into packets, as in batch melting,
before traveling away from the source?
3
How do melts travel through the melting regime? Do they travel as
independent melt pockets or do they pooi into a central column and travel
together to the crust?
According to the model of Sinton and Detrick (1992), once they reach the crust,
melt enter a partially molten transitional area known as the mush zone. Melts are thought
to percolate through this region in transit to the melt lens, or pooling area, in preparation
for eruption at the surface. It is here that magmas undergo significant mixing and
crystallization. However, similar to the those of the mantle, there are many unanswered
questions regarding processes in the crust. These include:
How much do magmas mix with one another in the crust?
How much do magmas fractionate?
Do magmas react with the wall rock en route to the surface?
How much of the original diversity of melt compositions produced in the
mantle is erupted at the surface?
Based on the two models above, some generalizations can be made about mid-
ocean ridges. At fast spreading ridges, spreading rates range from 80-130 mm/yr. These
ridges are generally underlain by hotter mantle, which produces a high magma flux,
thicker ocean crust, and the rise-type axial morphology. The high magma flux, in turn,
supports a steady-state melt lens. At slower spreading ridges, spreading rates vary from
0-50 mmlyr. There regions generally have cooler mantle temperatures, which lessens the
magma flux, produces less melt, and therefore thinner ocean crust and the valley-type
axial morphology. Slow spreading ridges do not have the flux necessary to support a
melt lens.
Although we recognize that each of these parameters plays an important role, to
understand where and to what extent each occurs, we must study the chemical
composition of MORB and their inclusions. In this thesis, I have undertaken a study of
lavas from three different mid-ocean ridge environments in an attempt to understand the
magmatic processes shaping each individual lava suite. In the first study, I address the
diversity of melt compositions formed as a result of partial melting using rehomogenized
plagioclase-hosted melt inclusions from the Endeavour Segment of the Juan de Fuca
4
Ridge. In the second study, I use rehomogenized melt inclusions in olivine and
plagioclase to determine the relationship between axial and seamount lavas from the
northern East Pacific Rise. For my third study, I undertake an evaluation of the current
paradigm of mid-ocean ridge dynamics using the Southeast Indian Ridge in order to
understand how the many different processes are inter-related.
Major and minor element concentrations of MORB lavas are used to model the
above processes. The difficulty is that each subsequent process alters the composition of
the melt, erasing or masking the chemical signature of the process before it. In order to
circumvent this problem, I have used melt inclusions found in MORB lavas. These
inclusions are small packets of melt which were incorporated into crystals at intermediate
stages in the history of the magma. As a result, in the majority of cases, these inclusions
represent more primitive (less processed) compositions than their MORB lava hosts, and
therefore, provide additional information not available with the lavas alone. When the
melt inclusions have the same origin as their lava counterparts, the same major and trace
element concentrations can be used to distinguish between processes.
In all cases, magmatic processes control the chemical compositions of the lavas
and melt inclusions erupted at mid-ocean ridges. Through these three studies, I have
found that:
In most cases, melt inclusions are related to their host lavas, and therefore are
a valuable tool to determine the diversity of melts produced.
Mid-ocean ridge basalts exhibit greater diversity on all scales than was
previously thought.
Global patterns exhibited by mid-ocean ridge basalt lavas have local
differences attributable to local variability in mantle temperature and
compositon.
5
LOCAL AND REGIONAL VARIATION OF MORB PARENT MAGMAS:
EVIDENCE FROM MELT INCLUSIONS FROM THE ENDEAVOUR SEGMENT
OF THE JUAN DE FUCA RIDGE
Rachel Sours-Page
Kevin T.M. Johnson
Roger L. Nielsen
Jill L. Karsten
MS published in Contributions to Mineralogy and Petrology
134: 342-363
July, 2000, 46 pages
6
Abstract
The development of petrogenetic models of igneous processes in the mantle is
dependent on a detailed knowledge of the diversity of magmas produced in the melting
regime. These primary magmas, however, undergo significant mixing and fractionation
during transport to the surface, destroying much of the evidence of their primary
diversity. To circumvent this problem and to determine the diversity of melts produced
in the mantle, we used melt inclusions hosted in primitive plagioclase phenocrysts from
eight mid-ocean ridge basalts from the axial and West Valleys of the Endeavour
Segment, Juan de Fuca Ridge. This area was selected for study because of the
demonstrated close association of enriched (E-MORB) lavas and incompatible element
enriched depleted (N-MORB) lavas (Karsten et al. 1990).
Rehomogenized melt inclusions from E-MORB, T-MORB, and N-MORB lavas
have been analyzed by electron and ion microprobe for major and trace elements. The
depleted and enriched lavas, as well as their melt inclusions, have very similar
compatible element concentrations (major elements, Sr, Ni and Cr). Inclusion
compositions are more primitive than, yet collinear with, the host lava suites. In contrast,
the minor and trace element characteristics of melt inclusions from depleted and enriched
lavas are different both in range and absolute concentration. N-MORB lavas contain both
depleted and enriched melt inclusions, and therefore exhibit the largest compositional
range (1(20: 0.01 to 0.4 wt %, P205: <0.01 to 0.2 wt %, LaN: 7 to 35, YbN: 1-13, and
Ti/Zr: < 100 to 1300). E-MORB lavas contain only enriched inclusions, and are therefore
relatively homogeneous (1(20: 0.32 to 0.9 wt %, P205: 0.02 to 0.35 wt %, LaN: 11-60,
YbN: 4-21, and TiIZr: 100). In addition, the most primitive E-32 inclusions are similar
in composition to the most enriched inclusions from the depleted hosts.
Major element data for melt inclusions from both N-MORB and E-MORB lavas
suggest that the magmas lie on a low pressure cotectic, consistent with a petrogenesis
including fractional crystallization. However, the minor and trace element compositions
in melt inclusions vary independently of the major element composition implying an
alternative history. When fractionation-corrected, inclusion compositions correlate with
7
their host glass composition. Hence, the degree of enrichment of the lavas is a function
of the composition of aggregated melts, not of processing in the upper mantle or lower
crust. Based on this fact, the lava suites are not produced from a single parent magma,
but from a suite of primary magmas. The chemistry of the melt inclusions from the
enriched lavas is consistent with a derivation from variable % partial melting within the
spinel stability field by a process of open system (continuous or critical) melting
assuming a depleted lherzolite source veined with clinopyroxenite. The low % melts are
dominantly enriched melts of the clinopyroxenite; In contrast, the depleted lavas were
created by melting of a harzburgite source, possibly fluxed with a fluid enriched in K, Ba
and the LREE. Such a source was likely melted up to or past the point at which all of its
clinopyroxene was consumed. This set of characteristics is consistent with a scenario by
which diverse melts produced at different depths travel through the melting regime to the
base of the crust without homogenizing en route. The homogeneous major element
characteristics are created in the lower crust by fractional crystallization and reaction with
lower crustal gabbros. Therefore, the degree of decoupling between major and trace
element characteristics of the melt inclusions (and lavas) is dictated by the reaction rate
of the melts with the materials in the conduit walls, as well as the residence times and
flux rate, in the upper mantle and lower crust.
Introduction
The diversity of melts produced in the mantle is one of the fundamental questions
in petrology. At the root of the problem is the fact that late stage processes such as
mixing, fractionation and reaction with cumulates obscure the chemical signature of
processes which take place early in the history of the magma system. In order to
understand these processes, we must find ways to see through the effects of crustal
processing and determine the compositions of melts at intermediate stages. Melt
inclusions provide much needed information to address this issue.
8
The several factors influencing the composition of a lava suite include the type
and degree of mantle melting, how the melts are extracted from the mantle, whether or
not they pooi and mix, and the extent of fractional crystallization (O'Hara and Mathews
1981; O'Hara 1985; Klein and Langmuir 1987; Defant and Nielsen 1990; Johnson and
Dick 1992; Kinzler and Grove 1992; Plank and Langmuir 1992). In order to decipher the
history of a lava and the physical nature of the environment in which it formed, it is
necessary to distinguish between the effects of these different processes.
One means by which the initial diversity of primary magmas can be preserved is
as melt inclusions in crystals formed prior to mixing and fractionation. Because of their
potential utility, melt inclusions have been an area of extensive work in recent years
(Watson 1976; Donaldson and Brown 1977; Dungan and Rhodes 1978; Falloon and
Green 1986; Hansteen 1991, Sobolev and Shimizu 1992, 1993, 1994; Shimizu 1994;
Sobolev et al. 1992, 1994; Sobolev and Chaussidon 1995; Nielsen et al. 1995a and
others). Ideally, each inclusion represents a "snapshot" of the liquid trapped during the
growth of the host crystal, preserving the composition of intermediate steps in the
evolution of a magma.
Although such liquids do not experience the same differentiation processes as
erupted lavas, they can be affected by post-entrapment crystallization. If quenched
immediately after entrapment, these liquids are preserved as homogeneous basaltic glass.
However, subsequent cooling in more evolved magmas and the development of quench
crystals at the time of eruption both affect the composition of the observed glasses
(Dungan and Rhodes 1978; Langmuir 1980; Vicenzi 1990; Sobolev and Shimizu 1993;
Sinton et al. 1993; Nielsen et al. 1995a,b; Johnson et al. 1996). Therefore, an
experimental means was developed for re-homogenizing the inclusions and evaluating
the relationship of the melt inclusion compositions with the observed range of parental
magmas and associated lavas (Nielsen et al. 1 995b) and for evaluating the level to which
the compositions represent the trapped magma (Nielsen et al. 1998).
Using the information provided by melt inclusions, we can address the roles of the
processes that affect MORB magmas from the time of their formation to eruption to
9
determine the diversity of primary melts formed in the lower crust/upper mantle. This
melt inclusion data set allows us to evaluate the following questions:
How diverse are the materials undergoing melting in the mantle?
Where are processes such as melt aggregation and homogenization taking place?
How are melt inclusions related to their host lava? How does the diversity of melt
inclusion compositions compare to the diversity of host lavas?
How does the frequency of melt inclusions at the surface relate to the volumetric
proportions of these same magmas produced in the mantle?
What process, or combination of processes, causes the extreme (over-) enrichment
trends in the incompatible elements of the lava suite?
In this paper, we will investigate the diversity of melts produced in the mantle,
and use that information to evaluate the local variability of melting mode and % melting
and to constrain the origin of the phenomenon of over-enrichment using plagioclasehosted melt inclusion data from lavas from the Endeavour Segment of the Juan de Fuca
Ridge. The Endeavour Segment was chosen for this study because it has erupted both
depleted and enriched lavas in close spatial and temporal proximity, allowing an
evaluation of the relationship between the above parameters and the formation of various
lava types.
The Phenomenon of Over-Enrichment
The phenomenon of incompatible element over-enrichment has been recognized
in many MORB suites (O'Hara and Matthews 1981; Newman et al. 1983; Hekinian and
Walker 1987; Frey et al. 1993; Gaetani et al. 1995). It is defined here as the systematic
enrichment of the incompatible elements as a function of decreasing Mg#, above levels
attributable to fractional crystallization processes alone. Over-enrichment has been
attributed to many processes including: boundary layer fractionation (Hekinian and
Walker 1987; Langmuir 1989; Nielsen and DeLong 1992), paired recharge and
fractionation (O'Hara and Mathews 1981; Nielsen 1990), assimilation (Michael and
10
Schilling 1989) and differing degrees of partial melting (Newman et al. 1983). However,
in spite of the attention given to this phenomenon, simulations of these processes have
not adequately reproduced the observed suites (Frey et al. 1993).
Over-enrichment is inherently difficult to model because it relates two divergent
characteristics, variable degrees of enrichment and Mg#, that are normally attributed to
the independent processes of differing degrees of partial melting and crystal (ol ± cpx)
fractionation, respectively. No known differentiation process can link the observed
degree of enrichment in the incompatible elements with a decrease in Mg#. Therefore,
over-enrichment must be the result of some unknown process or combination of
processes linking enrichment with Mg#.
The Use of Mg#
There is some question as to whether Mg# or MgO would be the best indicator of
differentiation. Due to differences in the compatibility of Fe and Mg, the Mg# of a melt
will depend on the degree of enrichment of its source material, as well as the % melting
These two factors may also affect the phase equilibria of the system, causing plagioclase
to crystallize at different times, preventing a direct comparison of Mg# in different
magmatic systems (Michael et al. 1989). Although MgO might be a better measure of
the absolute enrichment in the system, this component is more easily altered by small
temperature errors in the rehomogenization process, and therefore would result in
exaggerated scatter in the data. In this study, the normal and enriched lavas and melt
inclusions show variation in Mg# far greater than that potentially created by variation in
primary Fe/Mg ratio, and therefore, we use Mg# to describe the degree of differentiation
to which a magmas has been exposed.
11
Geologic Setting and Samples
The Endeavour Segment is the northernmost segment of the Juan de Fuca Ridge
in the northeast Pacific Ocean (Fig. 1). This segment is bounded by the Juan de Fuca
Ridge - Sovanco Fracture Zone - Nootka Fault Triple Junction in the north and the Cobb
Offset, an overlapping spreading center and the region of dueling propagation between
the Endeavour Segment and the next segment to the south (Karsten et al. 1990). It has an
average spreading rate of 29 mmlyr, half rate (Karsten et al. 1986). The geomorphology
of the area is dominated by an elongate crestal volcano, Endeavour Ridge, which has
been rifled apart to form a shallow axial valley. Paleo-ridges and valleys have formed
parallel to the ridge axis as a result of repeated increases and decreases in magma flux at
the ridge axis (Kappel and Ryan 1986). Tectonic reorganizations have also created ridges
and valleys. West Valley was formed when the spreading center jumped from Middle
Valley to West Valley, approximately 200,000 years ago (Karsten et al. 1986). In the
past 75,000 years, the spreading center has propagated southward from West Valley into
South West Valley, creating an overlapping spreading center with the North Endeavour
Valley (Karsten et al. 1986). The most recent volcanism in the West Valley has been in
the form of several small volcanic cones and a narrow rift zone in the center (Karsten et
al. 1986). These edifices all exhibit very young lavas. The samples from this study were
dredged from the northern end of the Endeavour Ridge, South Endeavour Valley, and an
adjacent abyssal hill (Karsten et al. 1990).
The Endeavour Segment lavas are moderately evolved basalts with Mg#s (Mg#
(Mg/(Mg+Fe))* 100) ranging from 48 to 63 (Karsten et al. 1990). This segment has
most recently erupted two coeval lava suites: an enriched suite and a depleted suite which
exhibits significant incompatible element over-enrichment (Fig. 2, 3). Karsten et al.
(1990) distinguished between the suites using the following criteria: enriched lavas had a
Zr/Nb<16 and a K20/P2O5>1.6, while transitional lavas were classified as those with
1 6<Zr/Nb<25. All samples with Zr/Nb>25 were considered to be depleted lavas. The
two lava suites exhibit nearly identical, overlapping compatible element trends (Fig. 2).
However, for some of the incompatible elements, particularly K20, the lava trends are
12
Figure 1 Batbymetry map for the Endeavour Segment, including sample locations. The
Endeavour Segment of the Juan de Fuca Ridge lies between the Cobb Offset and a triple
junction with the Nootka Fault and the Sovanco Fracture Zone. The present day axis
extends from the South Endeavour Valley (SEV) through the Endeavour Ridge and
northward into the North Endeavour Valley (NEV), where it is offset to West Valley.
Inset shows the study area relative to the northwest United States and Vancouver Island.
Figure 2 Representative major and minor element analyses of inclusions from
Endeavour Segment samples E-1 (open diamonds), and 0-2 (circles), E- 5 (pluses), and
E-32 (triangles). The dark shaded field represents the extent of the enriched host lavas,
while the light shaded field represents the extent of the depleted host lavas, using the data
of Karsten et al. (1990). White boxes represent the host lava composition. No intralaboratory calibration was performed for the lava and melt inclusion data sets. Dark lines
represent calculated fractional crystallization liquid lines of descent, with the final
percent crystallization noted. Primitive melt inclusion compositions were chosen to
represent the parent magmas. Note that the melt inclusions are generally more primitive
than, and collinear with, the host lava fields.
Figure 3 Representative minor element electron microprobe analyses of inclusions from
Endeavour Segment lavas. Samples, symbols, and liquid lines of descent are same as in
Fig. 2. Squares outlined in black represent the four host lavas from which these
inclusions were taken. Note that the majority of inclusions from the depleted hosts are
also depleted and that the inclusion population from the enriched host contain no depleted
inclusions. Note that for the minor elements, the inclusions are not always collinear with
the lava fields.
km
3.2
3.0
-2.8
-2.6
-2.4
"2.2
-2.0
1.8
1.6
-1.4
bathymetry
OO'N
30'N
48' DON
30'N
I 30'W
129'W
128'W
-1.2
-1.0
14
Figure 2
19
Enriched lavas
18 - 0 Hostlava
17 -
1654%
15 -62%
14 -70%
19
18 17 -
16-
54%
15 -62%
14 -70
19
18 17 54%
15 -62%
14 -70%
19
18 17 -
16-
54%
15 -62%
14 -709
13
40
45
50
55
60
Mg#
65
70
75
80
15
Figure 3a
1.0
Dpletcd lavas
0.8 -
Enriched lavas
59%
O Host lava
E-1
- 42%
C
0.6 -
0.4 0.2 - 70%
0.0
0-2
0.8 -
59%
- 42%
0.6-
-
0.4 -
0.2-
70%
0.0
E-5
- 59%
0.8 -
42%
0.6-
-
0.4 0.2
- 70%
0.0
E-32
A
0.8-
A
A
- 42%
0.6-
A
-
A
A
0.4 -
0.2- 70%
0.0
40
45
50
55
60
Mg#
65
70
75
80
16
Figure 3b
2.5
E-1
2.0
-
59%
0
0.5
2.5
I
42%
2.0
1.5
-
59%
I
:'
I
I
I
O-2
r
..
C
1.0
0.5
70%
-
.
2.5
I
2.0
42%
1.5
-
I
1E-5
59%
C
1.0
±
70%
0.5
2.5
E-32
2.0
- 42%
1.5
-
1.0
- 70%
4t
A
59%
C
0.5
0.0
;sds26717w
17
Figure 3c
40
45
50
55
60
Mg#
65
70
75
80
18
distinct, yet they converge at Mg# of 55 (Fig. 3). Of the eight samples in this study, six
are depleted (N-MORB), one is intermediate (T-MORB), and one is enriched (E-MORB).
The six depleted samples are very similar, and so we will describe further details of the
data for the two most characteristic of the depleted samples, E- 1 and 0-2, as well as the
transitional sample, E-5, and the enriched sample, E-32.
The samples selected for this study contain abundant (10-30%), large (2-20 mm)
plagioclase crystals, with relatively minor olivine and chromite. The plagioclase
phenocrysts exhibit a range of An8092. This strongly porphyritic rock type is widespread
in the mid-ocean ridge environment, but generally low in abundance in any particular
suite (AMAR - Frey et al. 1993; SEIR - Christie et al. 1995; AmsterdamlSt.Paul Douglas, 1998; Galapagos Platform - Sinton et al. 1993; FAMOUS - Langmuir et al.
1977; EPR - Hekinian and Walker, 1987 and Batiza et al. 1989; Chile Ridge - Sherman et
al. 1997; Gorda Ridge - Nielsen et al. 1995b and others). There are many advantages to
working with plagioclase. First, the presence of large numbers of melt inclusions in
plagioclase allows us to evaluate the diversity of melt inclusion compositions and
volumetric proportions of the parental melts (Nielsen et al. 1995a). Second, we can
constrain the melt inclusion entrapment temperature using the temperature at which
olivine daughter crystals, formed within the melt inclusion after its capture, melt back
into the glass. Since the inclusions contain both plagioclase and olivine, they may be
considered multiply saturated. Third, plagioclase has slow reaction rates relative to
olivine. And finally, because plagioclase has such low concentrations of the rare earth
and high field strength elements, it is unlikely that the host crystal would contaminate the
melt inclusion. However, there are limitations of using plagioclase-hosted melt
inclusions: (a) plagioclase phyric samples only tell the history of the magma beginning
with plagioclase-saturation; (b) plagioclase-phyric lavas are only a subset of the magmas
erupted and therefore must be treated with caution when comparing its characteristics to
those of an aphyric MORB.
19
Experimental and Analytical Procedure
Rehomogenization Technique
The crystals used for this study were removed from the sample after a coarse
crushing. Rehomogenization of the melt inclusions is performed by suspending
individual crystals by 0.003" thick Pt wire or a Pt "boat" in a 1-atmosphere gas mixing
furnace. Crystals are held at 1000°C for 20-30 minutes, and then heated to the
rehomogenization temperature for 2-3 hours. Basic phase equilibria and our experience
have demonstrated that the entrapment temperature is generally correlated with the
anorthite content of the feldspar host. The specific temperature is constrained by running
a set of incremental heating experiments at 100 intervals in the range of 1200° to 1270°C.
Electron Microprobe
Major element analyses were performed using the CAMECA SX-50 Electron
Microprobe at Oregon State University to determine the range, distribution and frequency
of melt compositions, as well as the variation in trace element content of the plagioclase
host crystal. In order to avoid over-sampling any specific magma type, analyses were
conducted using the grid system used in Nielsen et al. (1 995a). In this method, melt
inclusions were sampled within the plagioclase crystal at a 1 OOji spacing so that
inclusions from the entire crystal were represented. Analyses were performed using a
beam current of 30 nA, an accelerating voltage of 15 kV, and a defocused (3-5) beam.
Smithsonian standards, including USNM 113498/1 (Makaopuhi Lava) for Si, Al, Fe, Ca,
and Ti, USNM 133868 (Kakanui Anorthoclase) for Na, USNM 143966 (Microcline) for
K, and USNM 122142 (Kakanui Augite) for Mg, were used for glass calibrations
(Jarosewich et al. 1980). Na was counted first due to its susceptibility to beam damage
(Nielsen et al. 1995a). Major elements were counted for 10-20 seconds, while elements
in low concentrations, particularly P, required a counting time of 300 seconds. A subset
20
of these analyses were performed at even longer counting times. Analytical errors ranged
from <<1 oxide wt. % for Al203, MgO, Si02, 1(20 and CaO to 10 oxide wt. % for Cr203.
Using this technique, over 2000 melt inclusions were analyzed from eight samples.
Ion Microprobe
Based on an examination of the range of compositions determined by electron
microprobe, a set of large (>35 i) inclusions were. selected for subsequent trace element
analysis by secondary ion mass spectrometry (SIMS). These measurements were
conducted at the University of New Mexico/Sandia National Laboratories, the Woods
Hole Oceanographic Institution, and the Institute for the Study of the Earth's Interior in
Misasa, Japan, using CAMECA ims 4f, 3f, and Sf instruments, respectively. For these
measurements, a filtered 160 primary beam was accelerated through a 12.5kV potential.
A typical 4OnA beam was focused to a 10-35ji diameter spot. The secondary ions
produced from the bombardment of the sample were accelerated through a nominal
potential of 4.5kV, to which a 60-90V energy off-set had been applied. The energy
acceptance window was set at 30-SOy full width. The mass spectrometer was operated at
low mass resolution (MhM=320) in peak stepping mode which included 14 mass
stations ('875background, 30Si, 47Ti, 88Sr, 89Y, 90Zr, '37Ba, '39La, 140Ce, '42Pr, 14&Nd, '47Sm,
'51Eu, 153Eu, 163Dy, '67Er, '74Yb, '75Lu); secondary ions were detected using an electron
multiplier operated in pulse counting mode. Magnetic peak positions were calibrated
with volcanic glass standards. Absolute elemental concentrations were calculated by
comparing the observed metal/30Si1 ratios in the target to the same ratio as observed in a
basaltic glass standards (working curves). These standards were analyzed several times
each day. The external precision based on observations of the standard were in the range
of 3% (Ti, Sr, Zr) to 20% (Eu).
Data from the multiple laboratories were compared using a plot of Ti from the ion
probe versus Ti from the electron microprobe (Fig. 4). The data form a coherent 1:1
trend with a scatter of less than 13% suggesting that there is little or no intra-laboratory
21
Figure 4 Comparison of electron microprobe (EMP) and ion microprobe analyses of Ti
in melt inclusions illustrating that little or no intra-laboratory bias exists between the
different analytical facilities.
E-1
12000
O-2
+E-5
E-32
9000
E
I
6000
3000
0
3000
6000
9000
Ti (ppm) from EMP
12000
Table 1: Major and trace element analyses of melt inclusions from the Endeavour Segment lavas of the Juan de Fuca Ridge. Samples
E-1 and 0-2 are N-MORB, E-5 is a T-MORB, and E-32 is an E-MORB. An# represents the anorthite content (An#=[cation moles of
Cal(cation moles Ca+Na+K)]) of the host crystal in which the melt inclusion was entrapped. Rehomogenization temperatures are in
degrees Celsius and represent the maximum temperature to which plagioclase crystals were heated. Trace element data were collected
at three different facilities: the University of New Mexico/Sandia National Laboratory (UNM), Institute for the Study of the Earth's
Interior (ISEI) and Woods Hole Oceanographic Institution (WHOI). Rare earth elements are normalized according to values in
Anders and Grevesse (1989). All other trace elements are reported in ppm.
Saniple#
Host An#
Rehom.T
Lab
0-2-1
0-2-2
0-2-3
0-2-4
0-2-5
0-2-6
0-2-7
0-2-10
0-2-Il
0-2-12
0-2-13
0-2-14
0-2-IS
0-2-16
0-2-Il
0-2-18
88
88
88
88.5
88.5
88.5
0-2-8
89
0-2-9
89.5
89
89.5
90
90
1250
1250
1250
1250
1250
1250
1250
1250
1250
90
1250
90
1250
1250
1250
1250
90
1250
13MM
UNM
UNM
13MM
13MM
13MM
13MM
13MM
13MM
13MM
13MM
13MM
90
1250
UNM
90
1250
90
1250
13MM
UNM
13MM
UNM
50.22
49.93
49.52
0.80
49.58
0.58
0.76
0.51
16.65
16.38
16.32
7,99
8.05
16.26
8.38
9.05
0.16
9.23
0.13
9.75
8.52
0.10
10.80
13.59
13,41
13.09
1,52
2,15
2.42
0.04
0,04
0.05
100,75
0.15
0,05
0.01
100.00
67.06
67.33
93.7
87.3
18.5
12.5
37.3
45.0
3.2
3.3
6.5
1.7
1.9
2,2
2.3
2.5
3.2
0.0
0.0
0.0
3.7
Gd,,
0.0
0.0
4.9
5.9
0.0
Si0,
0.29
50.42
0.33
17,17
17.87
17.53
17.90
7.26
7,25
7.31
0.14
0.16
0.11
9.51
7.73
0.17
11.52
0.35
18.09
6.90
0.10
11.19
11.71
11.49
11.33
12.74
13.40
13.94
13.77
13.11
12.84
1.94
1.45
1.60
0.04
0.05
0.05
0.00
0.04
101.51
101.04
0.07
101.23
68.45
69.44
67.03
0.19
0.05
0.03
l02,38
72,65
0.12
0.04
0.07
2.43
0.04
0.03
1.14
0.03
2.27
0,03
98.1
83.8
73.7
84.3
20.7
17.5
12.8
18.9
27.1
44.6
39.2
4.6
3.1
3.2
3.2
1.9
21.7
4.3
3.1
2.8
3.8
2.6
4.0
0.0
0.0
0.0
4.5
0.0
5.7
0.0
6.8
0.0
0,0
7.2
6.8
7.3
7.2
6.7
0.0
0.0
0.0
0.0
0.0
5.2
5.8
0.0
6.5
0.0
3.6
0.0
0.0
6.5
8.0
9.1
52.24
51.61
49.68
50.32
49.34
50.74
TIO,,
50.83
0.55
0.62
0.80
17.29
17.10
17.43
17.73
FeO°
MnO
MgO
CaO
Na,,0
K20
8.55
6.95
7.40
0.13
6.88
0.1!
0.68
17.44
7.92
0.87
AI,,03
0.10
0.10
9.37
8.91
8.24
8.38
13.59
1,94
13.21
13.70
2.12
1.99
0.03
0.05
0.05
Total
0,08
100.68
0.09
0.05
0.03
I3.39
2.10
0.04
0.04
0.03
0.80
17.04
7.56
0.25
8.64
13.73
100.70
101.18
102.03
Mg!!
66.13
69.76
67.87
66.85
0.16
0.06
0.00
99.92
67.07
Sr
98.2
84.1
14.4
12.2
16.8
Zr
24.8
Ba
23.5
3.9
91.2
9.5
23.3
87.1
V
3.7
La,,
2.3
Ce,,
3.1
Pr,,
Cr,,0,,
49,93
50.23
0.32
49.10
0.55
2.93
0.08
0.05
0.11
UNM
49.68
0.32
18.09
7.17
0.16
49.96
0,31
50.27
0.35
17.88
17.55
7.36
7.11
0.13
0,12
11.41
11.52
11.68
13.07
13.08
13.21
13.14
1.67
1.56
1.44
l.57
0.05
0.21
0.01
0.02
102,16
0.06
101.87
0.05
102.12
0.13
101.73
73.31
74.23
73.70
0.04
101.69
74,54
0.12
0,04
0.03
101.99
73.63
0.04
0.02
0.13
0.00
80.0
97.2
95.8
96.0
96,0
80,5
13.0
6.1
6.9
6.8
6.1
6.8
29.1
17.2
4.3
1.7
1,5
4.0
5.0
3.4
1.9
3.1
3.3
3.5
4.1
1.8
2.5
1.7
1.7
3.2
2.0
2.5
1,7
2.3
2.1
2.3
0.0
0.0
3.9
6.2
0.0
0.0
0.0
0.0
6.1
4.2
7.7
7.1
0.0
0.0
3.4
7.0
0.0
7.9
0.0
7.4
0.0
0.0
0.0
5.7
0.0
8.2
6.1
3.1
0.0
5.6
0.0
8.0
0.0
0.0
3.6
2.2
0.0
0.0
2,9
7.5
0.0
3.6
0.0
0.14
73,94
0,0!
0.05
101,90
74,55
ppm
Nd,,
Sm,,
Eu,,
7.7
Dy,,
6.8
4.3
8.3
Er,,
0.0
0.0
6.4
0.0
0.0
8.4
0.0
0.0
Yb,,
5.8
4.7
0.0
0.0
7.6
0.0
7.7
Lu,,
5.4
0.0
0.0
5.7
0.0
0.0
0.0
0.0
0.0
0.0
Ti/Zr
161
141
150
131
113
142
125
126
167
155
178
0.0
450
0.511
0.693
0.501
0.477
0.468
0.642
0.515
0.590
0.403
0,554
0.466
0,525
0.462
0,601
0.502
0.495
0.437
0.502
0.518
0,682
0.532
0.707
(La/Sm)n
(Ce/Yb)n
0.418
0.512
0.0
8.7
0.0
4.5
3.1
86.3
2.5
90.6
6.9
3,0
3,4
1.2
1.4
1.4
1.3
1.6
1.8
1.8
1.7
0.0
0.0
3.6
0.0
0,0
0.0
0.0
2.8
0.0
0.0
3.6
0.0
0.0
2.8
7.1
5,7
6.1
6.3
0.0
4.0
0.0
2.8
0.0
3.0
0.0
2.4
0.0
0,0
3.4
0.0
2.8
0.0
589
0.474
0.802
3.6
0.0
3.0
0.0
0.0
0.0
0.0
1182
1280
471
383
0.627
0.52!
0.469
0.896
1.003
0.800
0.427
0.749
3.3
6.5
3.8
3.4
6.5
0.0
2.8
0.0
2.3
0.0
557
0,510
0.952
IN)
Table 1 (cont'd)
Sample#
Host An #
E-5-4
88.5
Rehom.T
1250
Lab
Si02
Ti02
Al203
FeO*
MnO
MgO
CaO
Na20
K20
P205
Cr203
Tothl
Mg#
E-5-5
88.5
1250
E-5-6
E-5-7
E-32-1
89
89
83
1250
1250
1230
E-32-2 E-32-3 E-32-4 E-32-5 E-32-6
86.5
90
90
90
89.5
1230
1230
1230
1230
1230
E-32-7
88.5
1230
E-32-8
88.5
1230
E-32-9 E-32-10 E-32-11 E-32-12 E-32-13 E-32-14 E-32-15
89.5
1230
87
1230
90
1230
88.5
1230
88.5
1230
88.5
1230
88
1230
UNM UNM UNM UNM UNM UNM UNM UNM UNM UNM UNM UNM UNM UNM UNM WHOI WHOI WHOI WHOL
50.68
51.01
51.22
0.83
0.68
50.77
1.24
1.45
1.86
1.84
16.14
7.23
0.16
9.07
14.62
2.17
15.45
15.87
15.56
15.65
7.11
0.11
9.01
14.17
9.04
0.33
7.72
12.90
2.94
8.99
0.18
7.62
12.73
2.92
15.64
8.72
0.19
7.73
11.84
0.46
0.15
0.00
101.43
60.35
0.41
50.23
0.11
0.07
50.79
2.44
0.43
0.15
0.00
0.05
100.68 100.33
69.09 69.30
0.16
0.04
100.74
60.18
2.82
0.68
0.34
0.00
8.98
0.45
7.18
12.32
2.85
50.77
0.93
16.02
6.97
0.69
9.27
12.43
2.31
0.67
0.29
0.46
0.01
0.07
100.02
70.33
100.83
61.25
101.46
279.5
30.0
149.6
n/a
30.6
26.2
0.0
23.0
17.4
276.4
58.76
0.11
50.51
49.59
1.91
1.17
15.54
15.37
7.45
0.86
16.92
7.33
0.13
8.83
13.78
51.01
9.08
0.12
5.83
11.56
2.69
0.68
0.30
0.02
98.75
53.36
0.09
8.70
13.08
2.40
2.70
0.37
0.14
0.06
51.27
1.02
15.67
7.12
0.20
9.13
51.20
0.99
50.94
1.64
15.78
15.68
6.80
0.14
8.95
8.60
0.18
7.72
12.75
12.75
11.77
2.29
2.38
0.46
2.70
0.57
99.67
70.12
99.36
67.55
100.71
68.20
0.44
0.18
0.04
100.12
69.54
244.7
33.0
159.8
n/a
32.0
28.1
237.7
20,5
83.8
n/a
198.2
253.6
249.0
16.4
19.7
18.1
61.0
n/a
73.7
n/a
15.4
13.7
79.4
n/a
20.0
13.1
11.3
0.0
0.41
0.18
0.01
0.17
0.04
0.28
0.03
100.09
61.53
50.58
0.99
15.72
7.01
0.18
8.77
12.89
2.56
0.52
50.13
0.90
51.11
15.62
7.65
0.29
9.35
12.59
2.32
15.69
0.51
1.07
7.20
0.15
9.31
13.36
2.55
0.39
0.19
51.89
1.10
16.06
7.21
0.11
9.36
12.98
2.56
0.40
0.20
0.18
0.13
0.01
99.41
69.04
0.01
0.04
0.04
99.50
68.54
101.07
69.75
101.90
175.6
19.0
53.7
59.5
9.8
n/a
n/a
n/a
n/a
20.9
n/a
n/a
n/a
n/a
23.4
9.0
0.0
0.0
14.2
16.1
0.0
8.5
10.0
0.0
12.2
8.9
0.0
0.0
69.82
50.57
1.24
51.00
15.57
16.12
8.26
0.19
9.19
12.85
2.54
7.54
0.15
9.24
0.50
0.26
0.02
101.20
66.47
0.70
13.76
2.42
0.39
0.09
0.02
101.44
68.60
ppm
Sr
158.0
159.2
V
11.5
Zr
14.2
41.1
Ba
La
Ce
23.6
7.9
6.8
Pr
0.0
Nda
Sm
0.0
6.2
5.6
34.0
23.7
6.4
213.0
25.1
80.5
50.7
14.6
197.7
27.6
91.4
51.8
15.3
31.1
147.5
195.7
18.0
n/a
56.7
n/a
30.1
11.8
65.6
79.9
18.2
139.0
n/a
27.8
16.0
14.3
24.3
12.7
0.0
0.0
16.6
14.9
9.6
0.0
0.0
23.5
9.5
0.0
26.1
12.3
0.0
10.6
13.4
12.7
0.0
22.4
18.3
8.6
18.7
9.3
7.7
10.6
9.5
16.2
0.0
0.0
9.9
15.5
7.4
14.9
11.5
8.0
5.4
12.9
10.2
8.2
0.0
0.0
0.0
0.0
0.0
0.0
0.0
8.5
15.7
10.2
0.0
8.9
0.0
14.9
0.0
9.6
0.0
0.0
14.3
6.2
0.0
7.5
12.9
8.1
9.5
8.9
0.0
6.6
0.0
0.0
0.0
0.0
0.0
8.1
7.4
12.4
7.0
0.0
8.4
5.1
0.0
0.0
89
2.033
2.383
93
1.615
108
1.916
2.073
0.0
79
1.850
2.350
0.0
91
0.0
90
2.055
2.315
12.9
14.2
0.0
0.0
0.0
0.0
0.0
0.0
5.1
12.1
11.8
12.2
18.6
11.3
Gd
Dy
0.0
6.8
5.2
Er
0.0
0.0
0.0
8.4
0.0
12.4
0.0
0.0
0.0
0.0
0.0
Yb
5.9
5.1
11.0
11.7
13.6
12.8
8.0
13.3
Lu
Ti/Zr
(La/Sm)n
0.0
120
1.364
0.0
0.0
0.0
80
0.0
75
(Ce/Yb)n
1.401
1.387
102
1.297
1.414
0.0
79
0.0
124
1.351
0.0
99
1.395
1.461
1.900
2.313
1.777
2.495
0.0
221.6
26.5
5.9
6.5
0.0
Eu
250.4
28.9
104
1.484
1.451
1.851
2.542
0.0
8.5
0.0
85
1.779
1.847
2.188
1.243
1.297
10.9
8.0
0.0
n/a
2.269
3.424
9.6
8.9
6.5
0.0
n/a
2.848
3.067
n/a
n/a
n/a
ala
25.5
18.0
ala
n/a
n/a
n/a
0.0
0.0
6.9
12.7
12.0
0.0
0.0
9.5
8.5
6.5
0.0
n/a
2.289
3.402
5.6
4.8
8.3
0.0
0.0
6.0
4.3
2.5
0.0
n/a
0.723
2.339
Table 1 (cont'd)
0-2-19
Sample#
HostAn#
Rehom.T
90
1250
Lab
UNM
52.69
Si02
Ti02
Al203
FeO*
MaO
MgO
CaO
Na20
K20
P205
Cr203
Total
Mg#
0-2-20
0-2-21
0-2-22
0-2-23
88.5
1250
1250
UNM
51.43
91.5
1250
ISEI
49.10
89.5
1250
ISEI
49.11
0.97
0.37
0.59
ISEI
50.02
0.48
14.77
17.91
17.13
17.02
10.33
14.08
6.26
0.15
10.85
13.30
8,60
0.17
9.24
13.67
2.05
1.66
1.81
0.01
0.05
0.08
0.05
0.06
0.02
101.64
58.66
0.05
102.45
59.33
0.36
0.02
0.03
99.99
75.55
7.58
0.18
9.43
13.62
1.98
0.04
77.9
7.7
2.7
5.2
2.8
3.6
0.0
94.1
0.58
14.53
9.91
0.24
7.89
13.48
2.20
0.10
88
0.20
8.45
0-2-24
88.5
1250
ISEI
50.29
0.45
17.08
7.51
0.12
9.23
13.54
1.95
E-1-1
86.5
1250
UNM
50.72
0.86
15.25
8.42
0.16
9.86
13.69
2.56
0.03
0.03
0.11
0.01
0.10
0.11
0.02
0.08
100.54
65.69
100.45
68.91
0.11
100.32
68.67
n/a
n/a
n/a
n/a
2.4
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
1.8
1.8
182.6
14.4
45.4
28.4
8.6
3.3
2.2
2,4
8.1
101.83
67.59
E-1-2
87.5
1250
UNM
52.03
n/a
15.24
n/a
n/a
7.96
n/a
2.54
0.05
0.09
n/a
n/a
n/a
E-1-6
87.5
E-1-3
E-1-4
87
1250
86
1250
UNM
50.70
E-1-5
87.5
1250
ISEI
50.29
1.47
0.50
0.52
E-1-7
84.5
1250
ISEI
50.23
0.85
16.98
16.93
16.95
16.49
8.71
7.32
0.16
8.00
13.78
2.33
0.14
7.20
0.17
7.94
8.44
0.16
6.78
13.22
2.29
UNM
51.81
0.79
15.74
9.03
0.16
8.14
13.56
2.54
0.19
0.06
0.02
102.04
61.63
0.14
7.31
14.04
2.15
0.21
0.13
0.02
101.85
59.94
1250
ISEI
49.93
13.99
2.41
0.14
E-1-8
88
1250
ISET
49.17
1.36
16.48
8.42
0.10
6.97
13.64
2.10
0.25
E-5-1
89
1250
UNM
50.50
0.72
E-5-2
89
1250
UNM
50.52
E-5-3
89
1250
UNM
51.09
0.73
15.75
8.40
0.21
9.61
14.20
2.18
0.68
15.80
8.15
13.55
2.25
16.23
8.36
0.21
9.39
12.96
2.67
0.09
0.35
0.14
0.00
101.54
66.69
0.29
0.12
140.8
15.8
E-1-9
88
1250
ISEI
49.94
0.80
16.69
7.41
0.07
8.30
0.09
99.33
66.28
0.22
0.10
0.03
98.80
58.87
0.06
98.69
59.60
0.05
0.03
99.18
66.61
n/a
n/a
n/a
n/a
3.3
n/a
n/a
n/a
n/a
9.3
n/a
n/a
n/a
n/a
8.2
n/a
n/a
n/a
n/a
5.6
3.9
8.7
10.1
4.7
9.1
11.5
5.6
5.5
8.6
12.7
6.3
15.6
6.9
0.01
0.01
0.09
99.56
66.06
0.16
0.14
9.37
14.14
2.19
0.24
0.11
0.01
102.00
67.11
0.00
101.92
67.20
132.1
124.7
15.5
ppm
Sr
Y
Zr
Ba
La
Ce
Pr
Nd
Sm
Eu
Gd
Dy
0.0
6.3
11.0
0.0
4.2
Er
Yb
Lu
Ti/Zr
(La/Sm)n
(Ce/Yb)n
22.5
47.8
3.8
3.3
5.3
0.0
0.0
n/a
n/a
n/a
n/a
1.4
2.0
2.3
3.3
2.8
178.8
142.4
15.0
17.0
16.5
4.8
168.4
42.1
111.1
28.4
9.4
162.0
7.8
13.7
20.6
3.9
5.1
0.0
9.9
0.0
0.0
4.0
0.0
0.0
4.8
3.7
3.5
0.0
33.0
75.2
29.9
9.7
9.9
0.0
0.0
6.9
5.0
7.1
11.4
6.7
14.6
4.1
4.7
5.7
4.3
3.0
3.3
0.0
0.0
35.6
26.3
5.4
5.6
0.0
0.0
15.9
31,5
n/a
3.7
4.2
28.5
n/a
3.6
4.0
0.0
0.0
6.6
6.6
5.9
0.0
5.5
6.7
5.4
7.6
9.3
8.7
13.0
9.2
10.7
7.8
5.7
8.6
6.4
10.4
5.1
6.6
7.9
0.0
8.0
2.2
4.6
7.4
5.5
4.2
3,9
0.0
0.0
0.0
0.0
4.2
6.7
14.7
8.7
0.0
10.9
3.3
7.5
5.3
7.1
14.8
3.8
7.0
18.2
0.0
2.3
6.3
4.8
0.0
0.0
6.9
0.0
18.9
0.0
2.2
0.0
4.4
4.5
3.5
6.7
17.1
7.3
6.6
7.6
0.0
9.7
2.9
5.8
6.1
3.8
6.1
15.6
6.1
0.0
20.8
0.0
6.2
0.0
4.0
3.6
6.4
20.2
5.9
6.8
7.3
0.0
6.4
0.0
4.2
0.0
0.0
0.0
0.0
100
n/a
0.629
n/a
1.164
1.669
n/a
0.567
0.617
6.5
n/a
0.880
0.0
114
1.324
1.624
0.0
219.7
18.5
n/a
0.0
84
0.697
0.585
7.0
n/a
0.383
0.436
0.0
250
0.781
1.043
2.9
126
6.4
n/a
0.373
0.693
4.7
1102
0.487
3.8
n/a
0.550
0.844
121
140
149
0.879
0.992
0.611
0.786
0.717
0.769
2.045
8.3
8.9
0.430
0.676
0.418
0.792
0.915
0.784
1.023
1.223
1.328
1.161
2.5
0.0
0.0
6.2
25
bias. Although there is a small systematic offset from a 1:1 line in the electron
microprobe data at high Ti concentrations, this offset is much less than the inherent
variation observed in the data.
Results
Major Elements
Both host lavas and melt inclusions from the Endeavour Segment of the Juan de
Fuca Ridge exhibit large, but similar variations in their major and compatible element
contents (Fig. 2; Table 1). The melt inclusions are generally more primitive than the host
lava suite, but they form a collinear liquid line of descent trend with them. Whereas the
lavas range from Mg# 45-65, the melt inclusions range from Mg# 54-76. Of the two
depleted host lavas, 0-2 is the more primitive, and contains melt inclusions with Mg#s
ranging from 65-77, while E-1 is the more evolved, containing inclusions with Mg#s
ranging from 58-70. The enriched host (E-32) is the most evolved with melt inclusions
ranging from 53-73, however the range of inclusion compositions within a single sample
often rivals the diversity of the lava suite as a whole.
Minor and Trace Elements
In contrast with the major elements, minor element abundances in melt inclusions
from the Endeavour Segment lavas exhibit a wide range of concentrations over the range
of Mg# and at any given Mg#. In general, both within a single sample and the suite as a
whole, the incompatible elements increase in concentration with decreasing Mg#. For
example, in a single sample 0-2, K20 varies between 0.02 and 0.42 wt %. Likewise,
Ti02, P205, and LaISm (not shown) also exhibit large compositional ranges reaching
maximum variations of a factor of 5, 11, and 13, respectively, in a single component at a
26
given Mg# for a single sample (Fig. 3). Although the absolute range of incompatible
element abundances is approximately the same, the samples show little overlap, and
instead plot as distinct and separate groups of data. In addition, while the minor and trace
elements may all be incompatible, they do not behave similarly and cannot be used as a
proxy for one another. Many of these characteristics are also observed in the host lava
suites, but only become evident when lavas erupted over the entire length of the ridge
segment are considered.
Generally speaking, the behavior of the trace elements can be predicted by that of the
minor elements. As one would expect, the inclusions from the enriched host exhibit the
most enriched concentrations in the LREE, while the inclusions from the depleted hosts
exhibit predominantly depleted patterns (Fig. 5, 6, 7). When all of the melt inclusions are
considered as a single chemical group, they exhibit a positive correlation between the
LREE and K20, Ti02, and P205 (Fig. 5) but a negative correlation with Mg# (not shown).
The HREE are positively correlated with Ti02 and P205 and negatively correlated with
Mg#. As a single group, the melt inclusions from the Endeavour lavas exhibit no
correlation of the HREE with K20 (Fig. 5). However, when the enriched and depleted
melt inclusions are considered separately, we find that the two groups exhibit distinct,
internally consistent trends that are unlike those of the suites considered together. Below
we will describe the range of characteristics for melt inclusions from the Endeavour
Segment lavas for each chemical group: N-MORB (0-2 and E-1), T-MORB (E-5) and EMORB (E-32).
N-MORB
Samples 0-2 and E- 1 both contain a large population of depleted inclusions and a
smaller population of enriched inclusions (Fig. 3). All depleted samples contain
inclusions with a wider range of K20, Ti02, and P205 than their own host. For K20, the
majority of inclusions cluster in groupings that span a wide range of Mg# and a small
range of K20 (Fig. 3). These groups form an en echelon trend which terminates at the
27
Figure 5 Fractionation corrected electron and ion microprobe trace element analyses of
inclusions from the Endeavour Segment lavas. Samples and symbols are the same as
those used in Fig. 2, with the addition of West Valley samples (plain circles). Inclusions
from the depleted lavas show a narrow range in their La concentrations and a wide range
in their Yb concentrations. In contrast, inclusions from the enriched lava show a wide
range of La values and a narrower range of Yb values.
Figure 6 Fractionation corrected ion microprobe analyses of inclusions from the
Endeavour Segment lavas. Samples and symbols are the same as those used in Fig. 2.
Several melting/mixing models are included: melting of a clinopyroxene-rich source
(filled diamonds), batch melting (X), fractional melting (*), open system melting of a
garnet source (light dashes), open system melting of a spinel source (dark dashes), and
mixing of enriched and depleted endmembers (open squares). Model parameters are
given in Table 2. Note that: (a) any one of these models could explain the correlation
between Ti and Zr; (b) The clinopyroxene-rich source model most accurately describes
the behavior of the inclusions from the enriched lava. Inclusions from the depleted lavas
are being buffered by a LREE, LIL-rich fluid which prevents then from being accurately
modeled by the spinel open system melting model; (c) The garnet open system melting
model clearly cannot be responsible for the behavior of the HREE in the enriched
inclusions.
Figure 7 Representative fractionation corrected, chondrite-normalized REE abundances
in inclusions from the Endeavour Segment lavas. Samples are the same as those used in
Fig. 2. Note the relatively large range of REE concentrations for the depleted inclusions
compared with the relatively narrow range of values for the enriched inclusions. Also
note that the depleted inclusions show a flat to depleted pattern, while the enriched
inclusions are LREE enriched.
28
Figure 5
40
16
E-1
35 - Oo 0-2
E-5
30_ A+ E-32
o West Valley
25
20_
15 -
12AAAA
A
AAA
A
A
Ak
10
t,
5I
0.00
14
A
AA
A
A
I
0.10
2- 0
+
+
I
0.20
0
I
0.30
0.40
0.50
0.00
I
0.10
0.20
K20
0.30
0.40
40
16
353025201510-
0 0
12-
AA
j
10 -
64-
D
0
0
+
i
i
0.2
20
i
0.4
0.6
0.8
1.0
0
I
I
0.4
0.2
Ti02
40
35
AA
e
I
0.0
o+4
A
8-
A
50.0
U
14 -
A
A0
0.50
1(20
0.6
0.8
1.0
hO2
16
0
14 -
30_
A
25
20
Ak
15
5-
D0
-
0
0.00
D
I
0.04
+
0
10 -
oc
4j
+
2_0
1
0.12
P2 0
AA
+
0.16
0.20
0.00
A
A
e
A
0
I
0.08
41A
AA
6-
A
10
12_
I
I
0.04
0.08
0.12
P205
0.16
0.20
8000
7000
6000
I
5000
4000
- cpx-rich source
- batch
fractional
OSM-gt
OSM-sp
D
Mixing
N-MORB
o
N-MORB
+ T-MORB
E-MORB
A
3000
2000
1000
10
20
30
40
Zr (ppm)
50
60
70
80
1400
cpx-rich source
-*- batch
1200
0
1000
80O
fractional
Mixing
OSM-gt
OSM-sp
o
0-2
A
E-32
E-1
E-5
o
+
I'
400
200
0
II
0
5
Yb
10
I
15
1400
X
-3&
-.
1200
0
1000
o
800
a
fractional
batch
cpx-rich source
Mixing
OSM-gt
OSM-sp
N-MORB
N-MORB
T-MORB
E-MORB
41
400
200
0
0.0
0.5
1.0
1.5
(La/Sm)n
2.0
2.5
32
Figure 7
100
100
-
E-1
0-2
E-5
100
E-32
1
La Ce Pr Nd Pr SmEu Gd Th Dy Ho Er TmYb Lu
33
trend of the depleted lavas. The total range of K20 concentrations in the melt inclusions
is 0.02-0.42 oxide wt. % for 0-2 and 0.08-0.55 oxide wt. % for E-l. Such groups are not
present for any other element (Fig. 2, 3). Instead, the majority of melt inclusions from
the two depleted hosts span a similar range of Ti02 (0.25-1.0 oxide wt. % for 0-2, 0.252.0 oxide wt. % for E-l) and P205 (<100 ppm -0.10 oxide wt. % for 0-2, <100 ppm 0.18 oxide wt. % for E-1), distinguished only by their offset in Mg# and the small
population of enriched inclusions found in E- 1. The most depleted melt inclusions have
concentrations that approach the P205 detection limits of the electron microprobe (--1 00
ppm) for the counting times and beam current used.
The melt inclusions from the depleted lavas exhibit diversity in their REE
concentrations beyond that exhibited by the T- and E-MORBs. 0-2 shows a range of a
factor of 2 in the LREE and a factor of>l0 in the HREE. Similarly, E-1 exhibits a range
of factors of 4 and 3 in LREE and HREE, respectively. In addition, the REE variation is
not uniform, with many of the melt inclusion REE patterns crossing. This is particularly
true of the inclusions from the most depleted host lavas.
Inclusions from the depleted lavas exhibit high levels of correlation between
HREE, P, and Ti. The LREE ratios (e.g. La/Sm) are correlated to the level of enrichment
of the host. For example, the average La/Sm for inclusions from the more depleted 0-2
is significantly lower than for the less depleted E- 1. For both depleted samples, the
La/Sm values of the inclusions do not drop below 0.4, even for compositions with
extremely low Ti, P and high Ti/Zr. In addition, the majority of inclusions from the
depleted group are characterized by a wide range of Yb values at constant Ti/Zr and
La/Sm. A subset of inclusions from 0-2 have an ultra-depleted character with
Ti/Zr>1200 at La/Sm0.4 (Fig. 6).
E-MORB
The E-MORB host, E-32, is the most evolved lava used in this study and, unlike
the depleted hosts, contains only inclusions enriched in K20 and the LREE. The most
34
primitive E-32 inclusions are similar in composition to the most enriched inclusions from
the depleted hosts. In E-32, K20 ranges from 0.32-0.90 oxide wt.%. The trend produced
by the E-32 inclusions in K20 vs. Mg# is parallel to the en echelon trend exhibited by the
groups of inclusions from the depleted hosts and the enriched lava suite. In contrast, the
melt inclusion Ti02 contents are collinear with both the enriched and depleted lava suite
trends and range from 0.4 to 2.0 oxide wt. % (Fig. 3). E-32 exhibits the most variation in
P205 of any of the inclusion populations studied, varying between 0.02 and 0.35 oxide
wt. % (Fig.3).
Compared to most MORB lava suites, the inclusions from E-32 are highly
variable, exhibiting a factor of 3 variation in K20. However, this variation is very small
relative to the factor of>20 variation in 1(20 exhibited by inclusions from the depleted
lavas. The E-32 inclusions are uniformly enriched in the LREE and Ba relative to the
depleted inclusions and they exhibit a generally enriched REE pattern with a narrow
range in both the LREE and the HREE (Fig. 7). While still greater than analytical error,
the HREE range is bracketed by that of inclusions from the two depleted lavas. In
addition, there is no apparent relationship between the HREE and any other incompatible
element (Fig. 5).
T-MORB
The T-MORB host displays characteristics common to both the N-MORB and the
E-MORB hosts. Melt inclusions from E-5 exhibit the most absolute variability in the
incompatible element contents of any of the samples studied. Inclusion compositions
show a more even distribution between the depleted and enriched compositions, than the
depleted and enriched samples. (Fig. 2, 3). For all elements, the E-5 inclusions are truly
transitional in the fact that they are bracketed by the depleted and enriched inclusions
from other hosts. K20 values range from 0.1 to 0.5 wt. %, Ti02 from 0.35-1.65 oxide wt.
%, and P205 from <100 ppm - 0.26 oxide wt. % (Fig. 3). The T-MORB REE patterns are
35
extremely narrow, with more diversity in the LREE than in the HREE (La ranging
between 5-11 ppm and Yb values between 8-9 ppm).
Effects of Over and Under-Heating During Rehomogenization
Over and under-heating of a feldspar host during the rehomogenization process
can have a significant effect on the chemistry of the entrapped melt inclusions.
Underheating occurs when the feldspar and inclusion have not been heated to a high
enough temperature to melt any post-entrapment crystals that may have formed. These
crystals should be visible in back scatter images, and therefore those melt inclusions that
contain them can be avoided. Over-heating occurs when the inclusion is heated to a
temperature at which the feldspar host begins to melt. As a result, over-heating will
cause the crystal and glass to begin to re-equilibrate. Elements that are found in high
concentrations in the feldspar, such as Al, Ca, Eu, and Sr will be particularly susceptible.
While these effects are significant, they are also isolated and predictable. Eu, for
example, shows an approximately equal number of positive and negative anomalies.
Even excluding data with extreme Eu anomalies, the melt inclusion trends remain the
same. This suggests that we are bracketing the rehomogenization temperature, and
therefore the composition of the inclusions. Most of the chemical variation exhibited by
the melt inclusions is seen in elements that are found in low concentrations in feldspar,
and therefore, could not be the result of re-equilibration with plagioclase.
The Relationship Between Melt Inclusions and Their Host Lavas
One of the basic tenets of melt inclusion research is that the trapped melts
represent the magma from which the phenocryst host was crystallizing at the time of
entrapment. If so, then MORBs should represent the aggregated sum of all these
included melt packets. One means by which this relationship can be explored is by
36
comparing the composition of the host lavas with the composition of melt inclusions
from each host. If the melt inclusions and the host are genetically related, and if the host
lava is derived from an aggregate of melts represented by the melt inclusions, then one
would expect to see a linear correlation between the two compositions after correction for
the effects of fractional crystallization. For the four samples in this study, we note that
there is a near 1:1 linear correlation between the concentration of K20 in the host and the
fractionation-corrected average K20 of all melt inclusions from that host (Fig. 8). The
effects of fractionation were removed by inverting the melt inclusion data using the
Ariskin et al. (1993) phase equilibria model (see fractionation correction section below).
All melt inclusions from a specific host were corrected to the MgO of their host lava.
Also included on this diagram is the range of melt inclusion K20 compositions
found within each host and the frequency distribution of each composition within that
range. No fewer than 150 microprobe analyses were used to determine the frequency
distribution for each host. In all cases, the distribution of inclusion compositions is
skewed toward the depleted end, with the extent of skewness greatest for the more
depleted hosts.
The offset of the averaged inclusion compositions and skewness towards the
depleted end of the compositional spectrum may be due to differences in phase equilibria
of enriched and depleted magmas (Edwards and Malpas 1996), or to the lack of
sensitivity of the average of the depleted melts to small numbers of enriched inclusions.
Only in the enriched lava does the range of inclusion compositions approach a normal
distribution, with no representatives of ultra-depleted magmas in the melt inclusion
population.
The fact that the hosts and their associated melt inclusions form a single trend and
that this trend is close to a 1:1 slope suggests that, in general, melts are being trapped and
analyzed in approximately the volumetric proportions in which they are created. For this
reason, it is of extreme importance to obtain a representative sampling of the melt
inclusions when developing the histogram. It is also important to note that a 1:1
correlation is not required for the results to define the characteristics or number of
37
Figure 8 Histogram representing the frequency of melt inclusion 1(20 values in each
host lava plotted against the host lava K20 composition. Arrows represent the entire
range of K20 values found in each host lava. Squares represent the average composition
of melt inclusions from a single host. Histograms show the relative frequency with
which a particular 1(20 composition appears in melt inclusions from that host. Dashed
line represents a 1:1 slope. Note that the depleted lavas have a greater proportion of
depleted inclusions, while the enriched lava shows a more normal compositional
distribution.
1:17
0.9 0.8 -
0.7 0.6 -
50.5C
E-1
0-2
0-I-0
0.1
0.2
0.3
0.4
0.5
0.6
K20* (MI)
* compensated forfract icnation
0.7
0.8
0.9
1
38
primary magmas. A simple linear correlation is sufficient to suggest that the lavas and
melt inclusions are genetically related.
Phase Equilibria Modeling
The main goal of this paper is to identify and quantify the processes shaping the
genesis of coexisting N-MORB and E-MORB lavas on the Endeavour Segment of the
Juan de Fuca Ridge. In particular, we would like to understand the origin of the
incompatible element over-enrichment trends exhibited by the N-MORB lavas. To this
end, we have used several different chemical models to try to identify those which best fit
the data.
Fractional crystallization
Fractional crystallization liquid lines of descent were determined using the models
of Nielsen (1990) and Ariskin et al. (1993). The major element concentrations of the
lavas and the melt inclusions from the Endeavour Segment accurately follow these
predicted curves (Fig. 2, 3). However, fractional crystallization does not accurately
predict the observed increase in LaJSm or other incompatible elements as a function of
MgO. Although groups of melt inclusion data from each host follow a fractionation
liquid line of descent, fractional crystallization cannot link the different groups together,
either within a single sample, or between samples. For example, the subparallel clusters
of inclusion data for 0-2 (Fig. 3) can each be attributed to fractionation, but one cannot
produce a trend linking the most primitive members of each cluster to one another by any
fractionation dominated process.
39
Fractionation correction
It is clear from the data, that fractional crystallization is an important factor in the
generation of the observed lava suites. In order to model the processes that may link the
parent magmas to their more evolved lavas, it is necessary to invert the fractionation
signature in order to obtain a composition closer to that of the parent magma (e.g. Klein
and Langmuir 1987). This fractionation correction was performed by using the phase
equilibria modeling program of Ariskin et al. (1993) to determine the amount of
fractionation as a function of Mg#. Because all of the inclusions from the Endeavour
Segment have undergone differing amounts of fractionation, we must calculate the
amount of fractional crystallization that would be required to produce that melt
composition from a parent magma with an Mg# of 75. That Mg# was selected on the
basis of the observed range of melt inclusion compositions, and the fact that a melt with
an Mg# of 75 is in equilibrium with Fo92, within the range of mantle olivine
compositions. This method assumes that Rayleigh fractionation is the only process
responsible for the change in Mg#. We recognize that there may be a significant
difference in Mg# between primary E and N-MORB. However, the observed wide range
of trace element contents and the primitive character of the melt inclusions suggests that
any error introduced by the assumption is comparatively small.
Melting Models
Fractional and Batch Melting and Mixing
Using the equations of Shaw (1970), we modeled the progressive melting of a
spinel lherzolite by fractional and batch melting to attempt to predict the behavior of the
trace elements. These models, as well as mixing curves, serve as a reference for the
discussion of the more complex models. The results of these preliminary calculations
40
demonstrate that such simple models do not adequately predict the behavior of the trace
element concentrations of melt inclusions from the Endeavour Segment.
Open System Melting
Based on the work of Johnson et al. (1990), Johnson and Dick (1992), and
Johnson and Kong (1992) we modeled open system melting in the garnet and spine!
stability fields with variable amounts of retained melt using the following equation:
(1P
-
I
Ci=Co (D0(1P)001 (D0+O)F(P+Ø)
(D0+Ø) (01)(P+ø)
(P+çb)
(1)
where C0 is the initial concentration of an element, C1 is the concentration in the melt
after F percent melting,
represents the weight percent melt retained in the pore spaces,
D0 is the bulk distribution coefficient of an element in the first moment of melting
weighted for the modal proportions of the source rock and P is the bulk distribution
coefficient of an element weighted for the proportions in which the existing mineral
phases melt. The partition coefficients, melt mode and initial concentrations were varied
to represent the conditions appropriate for the spinel or garnet stability fields (Table 2).
This model is conceptually similar to the critical melting model of Maaløe (1982), but it
allows for variation in the threshold porosity at grain boundaries and the introduction of
exotic melts into the system.
N-MORB
Varying degrees of open system melting of a spinel lherzolite source can
accurately reproduce the majority of trace element characteristics of the N-MORB melt
inclusions (Fig. 6). The degree of melting varies between 4-20% open system melting
41
assuming 2.5% retained melt. Because it is difficult to distinguish between the amount of
homogenization that takes place within incremental batches of melt versus that which
takes place later in the evolution of the magma, this retained melt value represents a
maximum. If more homogenization occurred later in the history of the magma, then less
of the melt need be pooled during melting. In contrast, this model fails to predict the
behavior of the LREE of the depleted melt inclusions, whose concentrations never drop
below 2 times chondrite or a La/Sm of 0.4. Likewise, the model predicts lower
concentrations of K20 and Ba than are found in the depleted inclusions. In effect, the
LREE, K20 and Ba appear to be "perched" at a significantly higher level than one would
predict from the HREE, Ti02 or P205.
One mechanism that would explain both the buffering of the LREE content, and
the high Ti/Zr content of the ultra-depleted inclusions is the presence of a fluid enriched
in K2O and LREE. This component would necessarily be low in Zr and Ti. Otherwise,
any interaction with the melt would drive the trend downward in Ti/Zr (see mixing curve
in Fig. 6). Due to the necessarily low concentrations of Ti and Zr, it is unlikely that the
LIL-rich component is an enriched basaltic magma as proposed by Elthon (1992) based
on information from abyssal peridotites. This is supported by the fact that the models
exhibit strong coherence on both Ti versus Zr and La versus Sm (Fig. 6).
E-MORB
The relatively wide range of LREE relative to the HREE contents exhibited by the
E- and T-MORB melt inclusions is consistent with observations of enriched MORBs
from other areas (Hirschmann and Stolper, 1996). Using partition coefficients and modal
mineralogies consistent with current literature (Table 2), the open system melting models
produce trends that exhibit greater variation in the HREE than observed in the melt
inclusions or the enriched lavas (Fig. 6).
The pattern of relatively low and constant HREE contents is one of several
chemical characteristics of E-MORB that have been attributed to a number of processes
42
including: residual garnet in the source (Bender et al. 1984; Frey et al. 1993; Shen and
Forsyth 1995), higher partition coefficients for pyroxene at low % melting (Gallahan and
Nielsen 1992), and preferential melting of clinopyroxenite veins (Hirschmann and
Stolper 1996). These arguments have been well summarized in Hirschmann and Stolper
(1996) and need not be reproduced here. In this case, melting of a garnet lherzolite
produces trends that exhibit extreme sensitivity to the amount of garnet in the source and
melting model (Fig. 6). For the Endeavour Segment inclusions, we have found that a
higher modal % of clinopyroxene in the source, together with the high partition
coefficients for a clinopyroxenite assemblage, such as those predicted by Gallahan and
Nielsen (1992) and Hack et al. (1994), has the effect of increasing D0 and P (Table 2),
thus alleviating the need for any garnet in the source material. This combination allows
the LREE to pivot on the HREE, creating greater diversity in the LREE compared to the
HREE (Fig. 7).
Discussion
The different lava compositions observed on the Endeavour Segment cannot
simply be the result of differing degrees of partial melting of the same mantle source. In
spite of the considerable overlap between inclusion compositions from the E- and NMORB host lavas, the absolute and relative ranges of inclusion compositions, the results
of open system modeling and the regional tectonics suggest that the enriched and
depleted lavas are derived from sources with different modal mineralogies, possibly at
different depths in the upper mantle (Fig. 9).
As suggested by their extremely low Zr values and their range in HREE
concentrations, the most depleted component in the array of N-MORB parent magmas
was produced from high degrees of near-fractional melting, at or near the point where
clinopyroxene is consumed. Clinopyroxene compositions in equilibrium with the
Endeavour inclusions can be calculated by multiplying the melt inclusion composition by
DCPX (Table 2). When compared with natural clinopyroxene compositions from abyssal
Figure 9 Cartoon representing the melting regimes necessary to create B- and N-MORB
lavas. Note that the two lava types require different sources, different extents of melting,
and different depths of melting.
enriched
/\
Clinopyroxenite
and Lherzolite
source
5
t
garnet
depleted
Harzburgite
source (cpx out)
spinel
,/
,/
garnet
44
Figure 10 Correlation of abyssal peridotite clinopyroxene analyses and calculated
fractional melting liquid line of descent from Johnson et al. (1990) with equilibrium
clinopyroxene, calculated from Endeavour Segment melt inclusion compositions which
have been fractionation corrected. The ellipses represent the range of composition for
abyssal peridotites from specific areas (Johnson et al. 1990). The most depleted ellipse
represents peridotites with as little as 1-3% modal clinopyroxene. Taken together, the
correlation of calculated % melting with modal % clinopyroxene (Johnson et al. 1990)
indicates that the most depleted melt inclusions were produced from a source that was
close to the point at which clinopyroxene was exhausted.
10000
Cpx in source
Ion probe data on Abyssal
Peridotite Cpx
Johnson et al. 1990
1000
Fractional
melting
=
A
25%
100
0.01
0.1
1
Zr in cpx (ppm)
10
100
45
Table 2. Modes, partition coefficients, and starting compositions used for trace element
modeling. Mode sources: Clinopyroxenite - modified after Hirschmann and Stolper
(1996); Garnet - Johnson et al. (1990); Spinel - modified after Kinzler and Grove (1992).
Partition coefficient sources: olivine - Nielsen et al. (1992); orthopyroxene - Johnson et
al. (1990) and Nielsen et al. (1992); clinopyroxene - Forsythe et al. (1994) and Hack et
al. (1994); spinel and garnet - Johnson et al. (1990) and Johnson (submitted). Starting
compositions are similar to those found in and were projected from the trend of the melt
inclusions. Modes are similar to those discussed in Johnson et al. (1990) and Hirschmann
and Stolper (1996). The clinopyroxenite mode was chosen arbitrarily. Partition
coefficients were determined from compositions of minerals and melt inthe high pressure
melting experiments of Takahashi et al. (1993).
Open System Melting of a Clinopyroxene.lich Source
InitialSolidMode(X)
MeltMode(P)
Ti
Zr
La
Ce
Nd
Sm
Yb
P
K
Ba
ol
0.15
-0.10
D
0.015
0.0005
0.00001
0.00001
0.00007
0.0007
0.023
0.00001
0.00001
0.00001
opx
0.15
0.38
D°
0.140
0.014
0.0009
0.0009
0.009
0.020
0.100
0.00001
0.00001
0.00001
cpx
0.65
0.67
D.
sp
0.05
0.05
D'
C0 (ppm)
1600
10
0.18
0.3
0.45
0.07
0.04
0.0006
0.0006
0.0006
0.0006
0.0045
0.0001
0.0001
0.0001
0
0
0
2.5
0.4
0.2
0.05
0.1
2
2.3
2.9
2.9
2.5
75
1.2
Garnet Open System Melting
0X
01
InitialSolidMode(X)
MeltMode(p)
0.55
0.13
D'
Ti
Zr
La
Ce
Nd
Sm
Yb
P
K
Ba
0.015
0.0005
0.000001
0.00001
0.00007
0.0007
0.023
0.00001
0.00001
0.00001
0.20
0.12
D'
CpX
Sp
0.15
0.25
D'
0.10
0.50
D
0.14
0.014
0.0005
0.0009
0.009
0.02
0.1
0.36
0.18
0.05
0.08
0.18
0.28
0.43
0.00001
0.00001
0.00001
0.0001
0.0001
0.0001
C0 (ppm)
0.6
0.5
0.0008
0.008
0.057
0.217
850
6
0.66
0.9
1.28
1
7
1
0
0
0
4
75
1.2
Spinel Open System, Batch and Fractional Melting Models
InitialSolidMode(X)
Melt Mode (P)
ol
0.56
-0.10
D"
Ti
Zr
La
Ce
Nd
Sm
Yb
P
K
Ba
opx
0.24
038
1)"
0.14
0.015
0.0005
0.014
0.00001
0.0009
0.00001
0.0009
0.009
0.00007
0.0007
0.02
0.023
0.1
0.00001 0.00001
0.00001 0.00001
0.00001 0.00001
cpx
0.16
0.67
D"
0.4
0.2
0.05
0.1
0.18
0.3
0.45
0.0001
0.0001
0.0001
sp
0.04
0.05
D
C0 (ppm)
0.07
0.04
0.0006
0.0006
0.0006
0.0006
0.0045
0
0
0
650
3
1
1.1
1.3
1.2
1.5
23
75
1.2
46
peridotites (Fig. 10), the clinopyroxenes in equilibrium with the most depleted melt
inclusions are equivalent to those found in abyssal peridotites with small percent (1-3%)
modal clinopyroxene (Johnson et al. 1990). This implies that the source is at or near the
point at which clinopyroxene is exhausted (harzburgite source).
Although our understanding of the changes in pyroxene chemistry at low modal
percent clinopyroxene is incomplete (Shimizu et al. 1997), we have a high level of
confidence that the partition coefficients for the HFSE and REE in clinopyroxene are a
function of Ca content (McKay 1989; Nielsen et al. 1992; Hack et al. 1994; Longhi and
Bertka 1996; Shimizu et al. 1997; Wood and Blundy 1997). In addition, we know that
clinopyroxene Dr is more sensitive to Ca content than DT (Forsythe et al. 1994).
Therefore, if the Ca content of the clinopyroxene drops as cpx-out is approached, Dr
would drop more rapidly than D11, resulting in a rapid increase in the Ti/Zr of the liquids
generated as Zr is increasingly fractionated from Ti. This scenario is consistent both with
the observed trends in the ultra-depleted melt inclusions and with the assumed original
modal composition of the mantle (Table 2). As such, at the 20%+ melting predicted by
the open system models, little if any clinopyroxene would remain in the source.
However, even if clinopyroxene does persist, the Ca content, and therefore the
composition, of that remaining clinopyroxene is unknown.
Based on the buffered HREE concentrations in the E-MORBs and the absence of
depleted inclusions, we suggest that the Endeavour E-MORBs are the result of melting of
a clinopyroxene-rich source that itself is a small component (<5% - Hirschmann and
Stolper, 1996) of the mantle as a whole (Fig. 9). Such a source is inferred to exist based
on the discovery of pyroxene-rich mantle xenoliths (Irving 1980; Frey 1980), as well the
existence of pyroxenité layers in ophiolites and alpine massif outcrops (Boudier and
Coleman 1981; Quick 1981; Suen and Frey 1987). This enriched source would have the
lowest melting point of any potential mantle source (Michael et al. 1989), and therefore
would be the first part of the upper mantle to melt. In addition, because the temperature
difference between the solidus and the liquidus of such a pyroxenite would be so small, it
would actually have a higher melt productivity than any surrounding peridotite
(Hirschmann and Stolper 1996). And, finally, because the pyroxenite is more enriched
47
than any surrounding material, it would likely melt faster than the peridotite (Hirschmann
and Stolper 1996).
However, if the E-MORB is derived from melting of clinopyroxenite, then the
total volume of E-MORB magma produced must be relatively small. Given the
spreading rate of this section of the Juan de Fuca Ridge, it is unlikely that the entire crust
could be constructed of enriched material. Karsten et al. (1990) reported that most
Endeavour Segment lavas reported from the axis between North Endeavour and South
Endeavour Valleys are enriched. If all the lavas on the surface are enriched, and if the
enriched lavas represent melts of a small percent of the mantle regime, then either the
enriched lavas must represent a thin veneer on an otherwise depleted crust, or the melting
regime under this area must be dramatically larger. These alternative hypotheses can be
tested by either a drilling project near the axis, or by sampling on an axial scarp.
The presence of enriched inclusions from the depleted host lavas suggests that
partial melts of clinopyroxenite may be a constituent in the array of magmas that
constitute the depleted parental magmas. However, the much greater volume of depleted
magma produced in the melting regime dominates the chemical signal. The limited range
of moderately incompatible elements and their evolved major element composition is
consistent with a model wherein the depleted melts produced at depth underwent
significant fractionation and mixing during transport through the upper mantle. In
contrast, the absence of any depleted material in the enriched lavas suggests that the
magmas did not re-equilibrate with depleted upper mantle clinopyroxene.
Several models have been put forth to explain the origin of E-MORB lavas in an
otherwise N-MORB environment. Hekinian et al. (1989) suggest that enriched lavas
from the northern East Pacific Rise are produced repeatedly during cyclic melting events.
E-MORB would be created at the inception of a new melting cycle and represent the first
few percent melting, when incompatible element concentrations are highest in the melt.
If the E-MORB from the Endeavour Segment were the result of a cyclic melting event,
one would expect to find E-MORB lavas from previous cycles on paleo-ridges. On the
Endeavour Segment, E-MORB lavas are only found at the current ridge axis, suggesting
that either E-MORB production is not a cyclical process in this location, or that previous
48
E-MORB eruptions have been buried by subsequent N-MORB eruptions (Karsten et al.
1990).
In contrast, Michael et al. (1989) suggest that enriched parental magmas found on
the Southern Explorer Ridge are derived from extensive melting of an incompatible
element enriched source, found near the top of the melting colunm. In this case, the
enriched source would represent a mantle heterogeneity, and the melting colunm would
produce less enriched melts in its deeper portions. This model is also inconsistent with
the melts found on the Endeavour Segment because the open system melting models on
the enriched melt inclusions have shown that these E-MORB melts were produced by
lower degrees of melting.
Karsten et al. (1990) suggest a model wherein E-MORB lavas are produced
during the final stages of a dying rift. At the Endeavour Segment, for example, depleted
magmas would be produced until spreading, and therefore the magma supply, was split
between two dueling propagators. As the Endeavour Segment becomes the failing rift, its
magma plumbing system has a cooler adiabatic gradient causing the degree of partial
melting to decrease and the depth of melting to increase (Fig. 9). The E-MORB lavas
produced at the Endeavour Segment would be the result of an increase in the enriched
component contributing to the final magma, caused by the decrease in magma supply at
the dying rift. Using the model of Karsten et al. (1990), the enriched lavas would
represent a veneer on top of an otherwise depleted crust. For this to occur, the enriched
magmas must pass through the depleted crust without significantly interacting with it.
Unlike the model of Klein and Langmuir (1987), our model suggests that, at least
in this tectonic setting, melts do not aggregate and homogenize completely within the
melting regime. Klein and Langmuir (1987) described a melting regime wherein partial
melts were created and aggregated prior to reaching the base of the crust. Because we
find distinctly enriched melts surviving at least to the depth at which plagioclase begins
to crystallize in the lower crust, these melts must be travelling through the upper mantle
in discrete packets, unaffected by any depleted material they may be travelling through.
The only way for the diverse chemistry represented in these melt inclusions to exist in the
lower crust is if they remain isolated from other depleted partial melts. This suggests
49
that, on the intermediate spreading Endeavour Segment, pooling and melt aggregation do
not begin until a depth shallower than plagioclase saturation, probably not until the
magmas reached the transient mid-crustal magma chamber.
None of these alternatives explains why we observe the correlation of enrichment
with decreasing Mg#. However, if one takes all the evidence presented above as a whole,
the composition of the component melts and the over-enriched trends can support a
working hypothesis wherein they are linked indirectly to mantle temperature. In effect,
areas characterized by component magmas that are dominantly depleted are most likely
to be areas where the upper mantle temperature has been relatively high and where
melting has persisted to high levels of the mantle. In such environments, residual
clinopyroxenite veins are unlikely to have survived to serve as a source for E-MORB
lavas. The composite magma from such areas would have less opportunity to fractionate
due to the higher flux rate and potentially higher temperature in the lower crust. The
opposite case would be true for areas where the composite magma is enriched.
Component magmas in those areas would have been produced by preferential melting of
clinopyroxenite in an environment that would produce less magma flux, and probably a
higher average % fractionation during transport through the cooler upper mantle and
lower crust.
Conclusions
The lavas of the Endeavour Segment and their associated melt inclusions are
genetically related. This contention is supported by the 1:1, linear relationship
between the mean inclusion composition and the host lava, and the fact that the trends
exhibited by the lavas and inclusions are collinear.
Over-enrichment trends are not due to the processing of depleted parental magmas.
Enriched, evolved members of over-enriched suites are enriched because they are
derived from primitive, enriched parental magmas.
50
N-MORB lavas formed from variable degrees of melting of a spine! peridotite. The
most depleted of the melt inclusions were the product of melting at or near the
clinopyroxene-out point (harzburgite source), possibly fluxed with a fluid enriched in
K, Ba and the LREE.
E-MORB lavas formed from melting of a source dominated by clinopyroxenite,
which itself represents a small % of the source. The production of the observed range
of REE patterns, characterized by a narrow range of HREE and wider range of LREE,
requires high partition coefficients for the HREE and a high clinopyroxene mode in
the source. Both are consistent with preferential melting of clinopyroxenite veins in
the spine! peridotite stability field, particularly at low % melting where the phase
compositions are consistent with high Ds. No residual garnet is required.
Melting of clinopyroxenite veins cannot produce the volume of magma required to
construct the full thickness of the oceanic crust. Therefore, the E-MORB lavas either
represent a veneer on an otherwise depleted crust, or the volume of the melting
regime is dramatically larger.
Melts travel through the upper mantle as discrete packets that do not aggregate prior
to passing into the base of the crust.
Acknowledgments
We wish to thank Dick Chase and Jamie Allen for their early guidance in sample
selection. Abigail Walker and Jennifer Crum were extremely helpful in providing
logistical and analytical support. Some of the ion microprobe data presented here were
measured at the University of New Mexico/Sandia National Laboratory SIMS facility, a
national multi-user facility supported in part by NSF grant number EAR 95-06611. K.J.
thanks E. Nakamura and I. Kushiro for providing the opportunity to carry out some of the
analyses in this paper at the ion probe facility at the Institute for the Study of the Earth's
Interior in Misasa, Japan. This work was supported by NSF grant OCE-9503782.
51
PARENTAL MAGMA DIVERSITY ON A FAST-SPREADING RIDGE:
EVIDENCE FROM OLI VINE AND PLAGIOCLASE-HOSTED MELT
INCLUSIONS IN AXIAL AND SEAMOUNT LAVAS FROM THE NORTHERN
EAST PACIFIC RISE
Rachel Sours-Page
Roger L. Nielsen
Rodey Batiza
MS in preparation for submission to Chemical Geology
July, 2000, 43 pages
52
Abstract
The axial plumbing system, and the axial magma chamber specifically, is
recognized as the site of much of the chemical processing that takes place at mid-ocean
ridges. Such processing is difficult to characterize, however, using only the lavas erupted
at the surface. To better understand the relative roles of different magmatic processes in
the formation of axial and off-axis magma systems, we studied plagioclase and olivinehosted melt inclusions from two axial segments, I O3O'N and 11 °20'N, and four
seamounts between 547'N and 9°17'N of the northern East Pacific Rise. These data are
a potential source of information on the characteristics of precursor magmas. By
comparing the compositions of inclusions with those of the host lava suite, one can
discern the nature of the intervening processes.
Rehomogenized melt inclusions from axial and seamount lavas have been
analyzed by electron and ion microprobe for major and trace elements. The lava groups,
as well as their inclusions, overlap and contain very similar N-MORB compatible and
incompatible element concentrations, with the exception of one N-MORB host lava
which contains both N- and E-MORB inclusions. Inclusion compositions are colinear
with and generally fall at the primitive end of the host lava suites. Inclusion diversity, i.e.
the chemical variation in melt inclusion compositions within a single sample, decreases
with decreasing MgO. Seamount lavas differ from axial lavas in that they are more
crystal rich and they contain a greater number of inclusions that are generally more
primitive, and they exhibit a larger compositional range in both the incompatible and
trace elements.
Axial and seamount lavas have comparable crystal assemblages and chemistry,
melt inclusion diversity and range of compositions, all of which corroborate what others
have suggested previously, that the axial and seamount magmas are formed in the same
mantle melting regime. We believe that axial and seamount magmas are created from a
single parent magma array derived by melting of a relatively homogeneous source. The
parent magmas undergo fractionation and mixing, then the axial and seamount magma
paths diverge. Axial lavas continue through the axial magma chamber, becoming more
53
fractionated and having more phenocrysts removed (possibly in the upper melt lens).
Seamount magmas travel to their volcanoes without further significant fractionation and
crystal separation.
The East Pacific Rise differs from slow and moderate spreading rate ridges
(AMAR, JdF, Gorda and SEIR) in its lack of phyric lavas, the scarcity of inclusions, the
lack of chemical variation between melt inclusions from the same crystal, the lack of
overall compositional variability, and the absence of E-MORB inclusions in the majority
of the N-MORB host lavas, suggesting that the EPR must have a fundamentally different
axial plumbing system.
Introduction
Melt inclusion data are currently emerging as a useful tool to interpret the
petrogenesis of basaltic magmas. Recent studies of mid-ocean ridge basalts, including
those from the Gorda and Juan de Fuca Ridges (Nielsen et al, 1 995b; Sours-Page et al.
1999; Smirnov 2000), the Galapagos region (Sinton et al. 1995; McNeil! and
Danyushevsky 199?), as well as the Mid-Atlantic Ridge (Sobolev et al. 1992; 1994;
Sobolev and Shimizu, 1993; 1994; Shimizu et al. 2000; Michael and McDonougb, 2000;
Tsameryan et al. 2000; Sobolev 2000), the Tongan Arc (Falloon and Green 1986), and
the Izu-Mariana Arc (Plank et al. 2000), have focused on areas where olivine and
plagioclase phyric lavas are most common. Through these studies, we have learned that,
like lavas erupted at mid-ocean ridges, melt inclusions vary in composition with
spreading rate, inferred mantle temperature, and crustal thickness. However, the regions
studied to date represent only slow and intermediate spreading ridges, leaving a large gap
in our knowledge of the behavior of melt inclusions on a global scale. For this reason, a
study of melt inclusions from a fast spreading environment is necessary.
The compositional diversity of melt inclusion populations reflects the effects of
processes that take place in the melting regime and the axial magma chamber (AMC).
Unlike their host lavas, such inclusions can provide pictures of the chemical composition
54
of a lava suite at intermediate steps in its petrogenesis, before the AMC processes destroy
the diversity characteristic of the parental magma array (Sours-Page et al. 1999; Nielsen
et al. 1995 a; Sobolev and Shimizu 1992, 1994). Processes that may have an important
role in the AMC include simple fractional crystallization, paired fractionation and
recharge (O'Hara and Mathews 1981; Defant and Nielsen 1988), boundary layer or in
situ fractionation (Langmuir 1989; Nielsen and Delong 1992), assimilation of altered
crust or mantle (Michael and Schilling 1989; Nielsen et al. 2000), and buffering reactions
with cumulates (Nielsen et al. 1995a; Natland and Dick 1996; Benoit et al. 1999). While
essentially all magmas are formed in the melting regime, they don't all pass through the
axial magma chamber. Therefore, it is how magmas experience the axial magma
chamber that will dictate the chemical diversity of the lava suites we see erupted at the
surface and limit what we can infer about mantle processes. Hence, it is critically
important to understand the intermediate steps in magmatic evolution.
Until now, published melt inclusion studies of mid-ocean ridges have focused on
axial volcanism, and used either plagioclase or olivine-bosted inclusions, but never both.
In this study, we compare olivine and plagioclase-hosted melt inclusions in both axial and
seamount N-MORB lavas from the moderately fast spreading northern East Pacific Rise
(EPR) to address the following:
How are on and off-axis magmas related?
How do olivine and plagioclase-hosted melt inclusions differ?
What is the diversity of primary magmas generated both on and off-axis in this
region?
Are melt inclusions from fast-spreading ridges different from those from slower
spreading ridges?
Is there a relationship between spreading rate and melt inclusion diversity?
55
Geological Setting
The northern East Pacific Rise (Fig 11) is a moderately fast spreading section of the
global mid-ocean ridge system with spreading rates ranging from 104 mmlyr in the north
to nearly 140 mnilyr, full rate, to the south of the study area (Niu and Batiza 1997). It
stretches from the Guaymas Basin in the north to the Galapagos Triple Junction in the
south, separating the Pacific from the Cocos Plate. This study focuses on lavas from the
segments at 1030'N and 1120'N, and seamoimts between 5°47'N and 917'N. We have
chosen this part of the EPR for investigation because it is one of the most extensively
studied portions of the global mid ocean ridge system. It has an abundance of near-axis
seamounts, adjacent to both magmatically robust (high flux) and starved (lower flux)
ridge segments (Regelous et al. 1999) and is unlike any of the other regions studied to
date using melt inclusions.
Axial Volcanism
The ridge axis is a volcanically active zone approximately 1 km wide (Batiza et
al. 1990). This zone is fed by a broad region of mantle upwelling, focused into the axis.
Seafloor compliance measurements have recorded low shear velocity zones at the base of
the crust and in the upper crust, indicating pooled melts in these locations (Crawford et
al. 1999). The region between the two melt lenses is a crystal mush zone containing 2.518% melt (Crawford et al. 1999).
In general, this area is characterized by homogeneous, fractionated, aphyric lavas,
erupting from a steady-state axial magma chamber. In detail, however, it is found that
the chemical characteristics of the lavas and the robustness of the axial magma chamber
varies along the ridge axis (Niu and Batiza 1991; Batiza and Niu 1992; Batiza et al.
1996).
56
Fig. 11 (a) Map of the northern East Pacific Rise. (b) Schematic map of the study area
showing ridge segments, isochrons, sample locations, and the distribution of previously
studies near-axis seamounts. After Niu and Batiza (1997).
57
Figure 11
110°W
20°N
i000w
1O°N
U°N
Ridge Axes
Overlapping
Spreading Centers
Transform Faults
Propagating Rifts
Absolute Plate
Motion Directions
Relative Plate
- Motion and
Spreading Rate
Crustal Isochrons
\
Previously Studied
Seamounts
Seamounts--This
Study
Axial Samples-This Study
103°W
102°W
58
Segment 1O°30'N
The 10°30'N segment is bound in the south by the Clipperton Fracture Zone at 10°15'N
and by the 1115 'N overlapping spreading center in the north (Fig 11) (Thompson et al.
1989). Its axial high and associated hydrothermal activity are centered at 10° 55 'N
(Thompson et al. 1985; McConachy et al. 1986). Regelous et al. (1999) show that supply
rates have changed significantly over time. However, the deep, narrow morphology and
lack of an AMC seismic reflector suggest that this segment is currently magmatically
starved compared to other regions of the northern EPR (Batiza et al. 1996). Computed
average eruption temperatures of 1060 ± 20°C confirm the starved nature of this segment,
as they are cooler than one would expect if there were a constant source of magma
flowing from below (Batiza et al. 1996). This segment differs from its neighbors by
erupting only N-MORB (Batiza et al. 1996).
Segment ll°20'N
The 11 °20'N segment is bounded by overlapping spreading centers at 11° l5'N
and 11°45'N, and its axial high and associated hydrothermal activity are centered at
1 1°30'N (Thompson et al. 1989; Hekinian et al. 1983, 1985). This segment is broad and
shallow and has a magma lens reflector suggesting the presence of a steady-state axial
melt lens (Detrick et al. 1987; Macdonald and Fox 1988; Macdonald 1989). The
computed average eruption temperature is 1190 ± 25°C (Batiza et al. 1996). Unlike its
neighbors, the ridge at 11 °20'N has erupted a large proportion of E-MORB, in addition to
N-MORB (Batiza et al. 1996).
59
Seamount Volcanism
Seamounts are the second form of zero-age volcanism near the East Pacific Rise
(Scheirer and Macdonald 1995). There are 179 seamounts greater than 200 m in height
on seafloor <2 Ma between 80 arid 17°N (Scheirer and Macdonald 1995). Their size and
abundance is correlated with the robustness (magma flux) of the ridge axis (Scheirer and
Macdonald 1995; White et al. 1998). Although seamount distribution is asymmetrical
(more on the western flank of the ridge axis), they form only in a narrow zone 5-15km
off-axis on crust 0.1-0.3 Ma, often adjacent to axial discontinuities (Scheirer and
Macdonald 1995; White et al. 1998).
The off-axis lavas used for this study are from searnounts located adjacent to
segments at 5°45'N, 8°30'N and 9°30'N. Each seamount is a single volcano potentially
related to the ridge axis. Petrogenetically, seamounts evolve from N-MORB, to
transitional and alkalic compositions over time (Batiza et al. 1989). However, many
become inactive before reaching the alkalic stage. When compared to axial lavas,
seamount lavas are systematically more primitive and heterogeneous, and sometimes
more depleted than those of the axis (Batiza et al. 1989).
Sample Information
Samples were chosen based on their plagioclase and olivine phenocryst contents
(we require 10-50 crystals >1mm per sample), location (samples from robust and less
robust segments), and lack of alteration (crystals need to break free from matrix upon
coarse crushing). Fewer than 1% of the lavas dredged from the axis and 5% of the lavas
from seamounts of the East Pacific Rise met our criteria, which made sample selection
very difficult. As a result, although our original intention was to select multiple samples
from adjacent axial segments and seamounts, sample availability required that we use
chemically and geographically similar, but spatially unrelated host lavas.
60
The ten samples used in this study (Table 3) include four axial lavas (two each
from the segments at 1 0°30'N and 11 °20'N) from the Phoenix cruise, leg 2 on the RJV
Melville, and six seamount samples from four seamounts from the RAIT cruise, leg 2 on
the R!V Thomas Washington (Fig 11). Axial host lava dredge locations, i.e. present-day
locations of lavas thought to have erupted within the axial neovolcanic zone, range from
approximately 10 km west to 10 km east of the spreading axis (Batiza et al. 1996).
Seamounts are located between 68 km west and 42 km east of the spreading axis
(numbers measured off map, assuming 111 km/° at this latitude). Based on their
K20/Ti02 ratios, nine of the host lavas are N-MORB, while only one, seamount sample
R32-5, is an E-MORB. The original works for the axial and seamount samples were
published in Batiza et al. (1996), Batiza et al. (1990) and Niu and Batiza (1997).
Experimental and Analytical Methods
Rehomogenization Technique
The crystals used for this study were removed from the sample after a coarse (0.5-
1 mm) crushing. In order to rehomogenize melt inclusions affected by post-entrapment
daughter crystal formation, host crystals were suspended by 0.003" thick Pt wire or a Pt
"boat" and heated in a 1-atmosphere gas mixing furnace. Plagioclase crystals were
heated in air, whereas olivine crystals were heated in a QFM-buffered environment.
Crystals were held at 1000°C for 15 minutes, and then heated to the rehomogenization
temperature for 45-60 minutes. Basic phase equilibria suggest, and our experience has
demonstrated, that the entrapment temperature is generally correlated with the anorthite
or fosterite content of the host mineral. The specific temperature is determined by
running a set of incremental heating experiments at 100 intervals in the range of 1170° to
1260°C. The appropriate rehomogenization temperature can be confirmed using phase
equilibria modeling (Johnson et al. 1996). If the calculated liquid line of descent is
colinear with, and falls within the trend of, the
61
Table 3. Major and trace element analyses of axial and seamount host lavas. Analytical
method as indicated. When glasses were available, major element analyses were
performed by electron microprobe. All other analyses were performed by either ICPMS,
published in Batiza et al. (1996) and Niu and Batiza (1997), or XRF, performed at
Franklin and Marshall College using the technique described in Boyd and Mertzman
(1987). When two analytical methods were used, an asterisk (*) indicates which
elements were obtained using the asterisked method. All major elements are reported as
oxide wt% and all trace elements as ppm. A number sign (#) after the analytical method
indicates that the analysis was performed on a sample from the same dredge, but not the
actual speciman used in this study. Rare earth elements are chondrite normalized using
the values reported in Anders and Grevesse (1989). Distances from the ridge axis are
approximate. Phenocryst major element analyses were performed by electron microprobe
to obtain anorthite and forsterite contents.
62
Table 3
AXIS
Host
Latitude('N)
Longitude ('W)
Averagedepth(m)
Dist from axis (km)
An# of fsp phenos
Fo# of ol phenos
SEAMOUNTS
--
PH55-8
P1165-I
PH93-3
P11113-4
R3-2
R3-4
R28-3
R28-7
R32-5
R34-3
10.48
103.53
3170
9.68 E
60-65
79
10.453
103.635
11.378
103.688
5.782
5.782
2932
10.05 E
102.2
1773
102.2
1773
8.812
103.9
1984
8.812
2996
3.39 W
66
9.085
104.9
3025
9.28
104.2
2611
86-89
84-89
80
76-88
-_
59
84-85
11.342
103.848
2931
7.86 W
78-86
85
--
__
--
--
88
70
82
ICPMS
50.79
ICPMS
50.52
1.90
15.10
10.17
0.19
6.94
11.72
2.92
0.20
0.20
99.86
57.49
XRF
EMP
49.12
0.91
XRF
50.03
EM?
50.97
2.16
EM?
EMP
49.80
XRF
50.74
15.81
0.54
16.68
8.10
0.15
7.11
11.82
2.68
0.20
0.17
100.00
53.85
9.70
12.74
2.16
0.03
0.12
99.74
68.12
2.41
0.06
0.16
1.47
16.59
8.05
0.16
8.63
11.64
2.74
0.34
0.24
99.99
62.94
99.76
66.04
99.76
65.64
1.67
14.42
10.82
0.20
7.32
11.82
2.74
0.12
0.15
100.00
54.67
ICPMS0
XRF. 1CPMS'
1CPMS
xItF, lC1'MS'
n/a
n/a
4.07
0.16
5.12*
0.21*
1CPMS
6.84
0.59
39
301
ICPMS
5.50
0.33
130
341
ICPMS
4.96
0.82
34
225
312
48
43
50
n/a
n/a
n/a
3.00
n/a
n/a
42.90
n/a
270.00
41.50
50.00
n/a
n/a
n/a
4.00
100
156
94
74
65
80
140
n/a
70
103.9
1984
Major element analyses (wt%)
Analytical Method
Si02
Ti02
Al203
Fe0
MnO
MgO
CaO
Na20
K20
P205
Total
Mg#
XRF
50.63
2.28
13.58
12.84
0.28
6.61
10.65
2.65
0.27
0.22
100.00
47.85
2.51
13.60
13.57
0.24
5.53
10.11
3.15
0.19
0.25
99.94
44.67
50.19
1.65
14.78
10.86
1.00
9.06
0.25
8.63
12.69
2.36
0.08
0.08
13.95
11.55
0.22
6.78
10.83
2.84
0.16
0.24
99.71
51.14
49.85
1.15
15.94
8.88
0.l9
8.72
12.41
Trace element analyses (ppm)
Analytical method XRF. ICPMS'
Li
n/a
Be
n/a
Sc
38
V
422
Cr
225
Co
41
Ni
72
Cu
53
Zn
109
Ga
20
Rb
3.10
Sr
103
Y
47
ICPMS
n/a
n/a
45
n/a
41
45
40
32
38
340
266
185
328
265
424
45
42
49
112
121
124
38
49
83
78
77
61
49
77
84
16
1.59
180.00
100
17
1.90
143
n/a
38
Zr
145
130
130.00
123
Nb
Cs
5.60
0.08*
54
12.02*
14.17*
n/a
16.91*
19.87*
18.50*
n/a
0.02
0
n/a
0.05
20.00
11.66
Ba
La.
Ce.
Pr,
Nd,
Sm,
Eu.
Gd,
Th,
Dy,
Ho,
Er,
Tm.
Yb,
Lu,
Hf
Ta
Pb
Th
U
Ti/Zr
La/Sm
n/a
20.56*
n/a
n/a
n/a
n/a
20.05*
20.08*
4.22*
0.30*
3.00
1.20
1.20
94
0.60*
n/a
n/a
5.00
0.01*
84
9.64
11.18
n/a
18.12
21.61
14.49
16.07
15.42
13.29
14.14
13.67
12.89
14.88
20.04
n/a
22.39
n/a
n/a
n/a
n/a
20.90
21.42
4.32
030
n/a
0.21
0.20
116
0.60
13.03
n/a
15.92
n/a
n/a
n/a
n/a
14.76
14.52
3.30
0.27
n/a
0.24
0.2
12.72
2.73*
0.26*
5.00
1.10
0.70
88
0.73
80
0.68
n/a
14.43
n/a
n/a
n/a
n/a
12.71
14
15
0.08
0.50
66
18.77
34.48
0.39
0.0025
1.27
3.87
5.53
7.08
8.64
11.00
12.55
12.13
12.10
12.25
12.05
12.14
11.63
11.53
11.11
1.12
0.03
0.16
0.03
0.01
158
0.35
78
27
62
3.40
0.0043*
50
2.45*
3.62*
499*
6.00*
8.04*
8.69*
9.04*
9.20*
945*
9.12*
9.16*
n/a
9.36*
9.19*
1.35*
0.041*
6.00
0.90
1.10
97
0.30
111
39.01
125.16
3.54
0.033
15.2
20.33
24.36
26.17
27.28
27.51
25.36
26.87
25.82
25.40
24.97
24.85
24.69
23.79
23.29
3.31
0.2418457
0.55
0.2695971
0.09436797
103
0.74
40
234
17
0.20
104
24.60
62.81
0.97
0.004
2.03
8.33
11.06
13.83
14.73
16.36
16.90
16.94
16.46
16.10
16.08
16.06
14.50
14.45
14.79
1.75
0.08007
0.30
0.056
0.03
110
0.51
17
7.17
219
24.16
113.10
8.88
0.12
76.18
35.80
34.46
30.84
27.87
22.81
21.69
19.79
18.03
16.81
15.79
15.10
14.65
14.30
13.79
2.65
0.54
0.78
0.76
0.24
78
1.57
XR.F. ICPMS
8.41*
0.48
43
352
212
40
74
77
94
17
0.30
121
37
114
4.80
0.013*
61
8.89*
11.21*
13.42*
14.50*
15.62*
15.41*
15.97*
15.82*
n/a
j459*
13.91*
13.91*
14.40*
n/a
n/a
0.19*
3.00
0.80
0.60
88
0.57*
63
host lava suite, and if the model indicates multiple saturation with plagioclase and
olivine, then we assume that the rehomogenization temperature was close to the
entrapment temperature (Sours-Page et al. 1999; Nielsen et al. 2000).
Effects of Over and Under-Heating During Rehomogenization
Over- or under-heating of a host mineral during rehomogenization can have a
significant effect on the composition of the melt inclusions. Underheating occurs when
the crystal and inclusion have not been heated to a high enough temperature to melt any
post-entrapment crystals that may have formed. These crystals are visible in back scatter
images, and therefore those melt inclusions that contain them can be avoided. Overheating occurs when the inclusion is heated to a temperature at which the mineral host
begins to melt, causing dilution of the trapped melt composition. The technique
described above minimizes this possibility by setting the temperature of rehomogenization to the crystal composition, not to the disappearance of a shrinkage
bubble, as in other rehomogenization techniques.
Elements in high concentrations in the host will be particularly susceptible to the
effects of over (or under) heating. For plagioclase, these include Al, Eu, and Sr. For
olivine, these include Mg, Fe, and Ni. While these effects are significant, they are also
isolated and predictable. Most of the chemical variation exhibited by the melt inclusions
is seen in elements that are found in low concentrations in the host crystal, and hence,
should not be the result of host crystal re-equilibration (Table 4).
Electron Microprobe
Major element analyses of melt inclusions and host lavas were performed using
the CAMECA SX-50 Electron Microprobe at Oregon State University to determine the
range, distribution and frequency of melt compositions. Analyses were performed using
64
a 30 nA beam current, 15 kV accelerating voltage, and defocused (5pm) beam.
Standards, including USNM 113498/1 (Makaopuhi Lava) for Si, Al, Fe, Ca, and Ti,
USNM 133868 (Kakanui Anorthoclase) for Na, USNM 143966 (Microcline) for K, and
USNM 122142 (Kakanui Augite) for Mg, were used for glass calibrations. Na was
counted first due to its susceptibility to beam damage (Nielsen et al. 1995b). Major
elements were counted for 10-20 seconds, while elements in low concentrations,
particularly K, P, Cl, and 5, required counting times of 100, 100, 500, and 50 seconds,
respectively. Due to low concentrations in the glasses, the primary standard for Cl,
Tugtapite (7 wt % Cl), was cross-standardized with Scapolite (USNHMR6600-l).
Chalcopyrite (Taylor 5-3) was used as the S standard.
Ion Microprobe
All melt inclusions of sufficient size (>35iim) and available lava glass chips were
subsequently analyzed for trace elements by secondary ion mass spectrometry (SIMS).
These measurements were conducted at the University of New Mexico/Sandia National
Laboratories using a CAMECA ims 4f instrument. For this procedure, a filtered
160
primary beam was accelerated through a 12.5kV potential. Typically, a 4OnA beam was
focused to a 10-35im diameter spot. The secondary ionsproduced from the
bombardment of the sample were accelerated through a nominal potential of 4.5kV, to
which a 60-90V energy off-set had been applied. The energy acceptance window was set
at 30-50V full width. The mass spectrometer was operated at low mass resolution
(M/EM=320) in peak stepping mode which included 14 mass stations ('875background,
30Si, 47Ti, 88Sr, 89Y, 90Zr, '37Ba, '39La, '40Ce, 147Sm, '5tEu, '53Eu, 163Dy, '67Er, '74Yb)
secondary ions were detected using an electron multiplier operated in pulse counting
mode. Magnetic peak positions were calibrated with volcanic glass standards. Absolute
elemental concentrations were calculated by comparing the observed metal/30Si ratios
in the target to the same ratio as observed in a basaltic glass standards (working curves).
65
These standards were analyzed two or more times each day. The external precision based
on observations of the standard were in the range of 3% (Ti, Sr, Zr) to 20% (Eu).
Results
Mineralogy
The northern East Pacific Rise erupts lavas that range from aphanitic to sparsely
phyric (<5% crystals all <2 mm). The mineral assemblage varies from plagioclase or
olivine to plagioclase + olivine +7- clinopyroxene. We present information for 10
samples found to bear analyzable glass inclusions. Their feldspar phenocrysts range in
composition from An8960 and olivine from Fo8878 (Fig 12a; Table 3). The seamounts
contain more crystals (< 10%), but are otherwise mineralogically similar.
Melt Inclusion Major and Trace Element Behavior
N-MORE lavas and melt inclusions from the axis and seamounts of northern East
Pacific Rise exhibit overlapping, subparallel trends in all the major elements (Fig 13). As
in most basaltic suites, K20, Ti02, P205, FeO, and Zr increase and Al203 and CaO
decrease with decreasing MgO (Fig 13). As a group, the melt inclusions are more
primitive than the lava suite. Seamount lavas and inclusions are generally more
magnesian than their axial counterparts, with lavas and inclusions ranging from 9.70-7.32
and 12.04-7.60 wt % MgO, respectively (Fig 13; Table 4, 5). Two of the seamounts are
represented by two samples each. In both cases, the melt inclusion compositions from
the two samples are nearly identical. The axial lavas and their associated melt inclusions
range from 7.11-6.61 and 9.11-4.66 wt % MgO, respectively. Although the sampling is
limited, in neither axial nor seamount samples do we see systematic differences in the
host or inclusions of samples from different latitudes.
66
Fig 12 Host crystal and lava compositions versus associated melt inclusion MgO. Black
symbols represent olivine-hosted inclusions, while gray symbols represent plagioclasehosted inclusions. In (a), note that melt inclusion compositions are correlated with
olivine and plagioclase compositions, and the more primitive crystals generally host the
more primitive melt inclusions. In (b), note that the range of melt inclusion compositions
(AMgO) varies between samples and that the range approximates a 1:1 line.
Fig. 13 Major and minor elements versus MgO for representative inclusions from axial
(a) and seamount (b) lavas from the East Pacific Rise (not all samples are shown for
purposes of clarity). Each symbol represents inclusions in either plagioclase (fsp) or
olivine (01) from a particular lava. Shaded fields represent the compositional extent of
glass separates from the lavas of segments 10°30'N (light Gray) and 1120'N (medium
Gray), using data from Batiza et al (1996), excluding the E-MORB (KITi> 0.15) and
FeTi basalts (FeO*> 12 wt %, Ti02 > 2 wt %). White boxes with symbols enclosed
represent the host lava compositions. No intra-laboratory calibration was performed for
the lava and melt inclusion data sets. Note that the melt inclusions are generally similar
in composition to the host lava suites and that diversity of inclusion compositions
decreases with decreasing MgO. One data point differs from all others and yet, seems
analytically sound. This inclusion, from PH 113-4, is primitive with MgO 10.44, but
falls within the trend for CaO and Na20. It has unusually low SiO2 45.85, and high
Ti02 = 3.63 and 1(20 = 0.33. While this point is definitely different from all of the other
inclusions analyzed, we could not find reason to exclude it. Therefore, on figures for
which this datum does not fit within the scale of the other inclusions, we have indicated
where it would fall with an arrow and the element values in parentheses.
67
Figure 12
95
0
cj OOO)1(
85
XX
0
0
V
XX
. -a:..-
80
+
-1+
:75
LPH933
-R34-3
£PH93-3
R3-4
A
R28-7
ol
ol
XR32-5
ol
PH113-4
£
70
+PHS5-8 ol
OPH113-4 ol
fsp
fsp 0R3-2 fsp
DR2S-3
fsp
fsp
fsp
A
65
4
5
6
7
10
9
8
11
12
13
MI MgO
10.0
-:
9.0 -
xm0*< X XX
8.5 8.0 7.5
0
7.0 6.5 6.0
5.5
5.0
4
S
6
7
9
8
MI MgO
10
11
12
13
68
Figure 13a
4.0
oAxialglass
± PH55-8 ol MI
PH93-3 ol MI
A PH93-3 fsp MI
3.5
PHI 13-4 fsp MI
PH55-8 Host
Li PH93-3 Host
OPHI134oIMJ !PH113-4Host
.
2.0
0
1.51.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
A
4
A
0
I
2.5
2.0
A
1.5
1.0
0
0.5
0.0
45
6
7
8
9
MgO
10
11
12 13
69
Figure 13b
4.0
0 Seamount Lavas
o R3-2 fsp MI
ri R3-2 Host
JR3-4FIost
.R3-4fspMI
R32-5 Host x R32-5 ol ME
8 R34-3 Host - R34-3 ol MT
3.5
3.0
C
z
2.0
1.5
1.0
0.9 0.8 0.7
0.6 -
CO.50.4-
X
0.3 0.2
0.1 -
XX
X
0.0
25
2.0 -
C
15
X
X
0.5
0.0
45
6
7
8
9
MgO
10
11
12 13
70
Table 4 Major and minor element analyses of melt inclusions from axial and seamount
lavas from the northern East Pacific Rise. All analyses were performed by electron
microprobe and are reported as oxide wt %, except S and Cl, which are reported as ppm.
Host refers to the mineral host of the inclusion, either olivine or plagioclase. Fo/An# is
the fosterite or anorthite composition of the host crystal, where Fo#=(cations
Mg)/(cations Mg+cations Fe) and An#=(cations Ca)/(cations Ca+cations Na+cations K).
Temp represents the temperature in degrees Celsius to which the host crystal was heated
in order to rehomogenize the melt inclusions. Size refers to the size of the individual
melt inclusion in microns.
Table 5 Trace element analyses of selected melt inclusions from axial and seamount
lavas from the northern East Pacific Rise. All analyses were performed by ion
microprobe and are reported as ppm. Rare earth elements are chondrite normalized using
the values reported in Anders and Grevesse (1989).
Table 4.
Label
host
Fo/An#
Temp
Size
P1155-8-a P1155-8-7
P1155-8-8
p1155-8-9 P8155-8-tO P1155-8-Il P1155-8-12 P1155-8-IS P1155-8-14 P1155-8-1511455-8-16 P1155-8-17 P1155-8-18 P1155-8-19 P1155-8-2011155-8-2! P1155-8-22 P1155-8-23 P1155-8-24
P1155-8-4
P1155-8-5
ol
ol
ol
ol
ol
ol
ol
ol
ol
ol
ol
ol
ol
ol
ol
ol
ol
ol
ol
ol
ol
ol
ol
ol
79
79
79
79
80
78
78
80
79
79
79
78
78
79
79
79
80
80
80
79
79
80
81
1170
1130
1170
1170
1170
1170
1170
1170
79
1170
1170
1170
1170
1170
1170
1170
1170
1170
1170
1170
1170
1170
1170
1170
1170
20
6
20
24
18
12
8
30
11
5
6
12
20
20
12
12
20
36
17
28
45
16
27
Il
50.71
50.32
49.76
49.41
51.06
49.99
50.02
50.92
49.33
2.23
49.99
49.62
51.06
50.92
50.03
49.18
49.28
49.06
49.22
48.90
48.76
50.10
49.85
P1155-8-I P1155-8-2 P1155-8-3
2.36
2.64
2.47
2.22
1.65
2.31
2.45
2.28
49.50
2.25
2.23
2.21
0.90
0.94
2.09
1.97
2.14
2.07
2.08
2.09
49.44
2.19
2.17
2.24
1.47
Al203
13.93
8.76
12.97
13.72
15.07
13.44
13.50
14.27
13.55
13.48
13.63
13.18
17.90
17.48
13.26
12.75
13.86
13.98
14.17
13.99
13.84
13.32
14.18
14.62
Fe06
12.23
12.97
13.91
12.20
11.45
13.94
14.02
10.90
13.11
13.31
13.14
13.24
7.56
7.80
13.59
13.64
12.03
11.96
11.77
11.79
11.93
13.00
12.01
11.73
MnO
MgO
Ca0
Na20
0.22
0.29
0.27
0.16
0.20
0.25
0.23
0.19
0.21
0.21
0.20
0.21
0.14
0.12
0.21
0.21
0.20
0.20
0.20
0.22
0.21
0.24
0.21
0.18
7.03
11.56
6.52
6.53
6.84
6.68
6.60
6.59
6.80
6.75
6.61
6.58
7.48
7.59
6.59
8.38
6.79
6.97
6.80
6.97
6.86
6.80
6.86
6.87
10.67
11.32
10.02
10.55
11.60
10.94
10.43
11.31
10.26
10.16
10.34
10.27
12.90
13.00
9.94
9.79
10.55
10.80
10.97
10.98
10.78
11.29
11.15
11.64
2.85
0.18
1.52
2.95
0.25
2.62
2.78
2.68
2.96
2.79
2.84
2.80
2.83
2.22
2.82
2.67
2.67
2.74
2.77
2.31
2.81
2.65
0.13
0.15
0.18
0.21
0.07
0.06
0.21
0.17
0.15
0.21
0.14
0.21
0.23
0.22
0.22
0.21
0.09
0.26
0.19
0.21
0.20
0.16
0.22
0.22
0.10
0.13
0.03
0.04
0.02
0.08
0.04
0.19
0.23
0.16
0.22
0.17
0.23
0.15
0.30
0.19
0.21
0.19
0.21
0.24
0.26
2.44
0.16
2.72
0.16
2.68
0.17
2.37
0.15
0.03
0.03
0.04
98.49 100.14
Si02
Ti02
K20
P205
Cr203
Total
Mg#
Sppm
Cl ppm
Ct/K
P/Ti
98.22
50.95
0.03
98.47
50.71
51.30
0.02
98.64
50.62
48.23
50.45
99.43
51.07
1438
1370
1336
1481
1551
1539
1393
322
0.23
313
0.24
311
304
341
335
314
108
0.23
0.24
0.25
0.24
0.13
0.10
0.10
0.09
0.11
0.10
0.22
0.10
0.10
0.10
0.09
R28.71
1128-7-2
1128.7.3
1128.7.4
1128-7-5
1128-7-6
1628-7-7
1628-7-8
1628-7.9
1128-7-tO
plag
plag
plag
plag
plag
plag
plag
plag
plag
plag
87
87
76
1220
1220
1220
1220
1230
1230
1230
1230
1230
1230
1230
20
28
49.43
10
24
12
12
15
82
40
14
50.50
49.52
49.78
49.77
49.68
49.95
49.77
50.24
0.80
24
49.28
0.82
0.90
0.44
0.58
0.60
0.54
0.31
16.23
16.40
0.73
16.18
0.37
16.34
16.41
16.38
16.16
16.29
16.29
19.42
8.22
0.16
8.06
0.16
8.41
8.39
0.15
0.16
7.40
0.17
0.02
0.06
0.00
0.03
0.03
0.02
0.03
0.02
0.03
0.02
0.04
99.96
99.54
98.02 101.10 100.80 100.99
99.87
0.03
99.17
0.02
100.62
98.87
99.58
98.59 100.60 100.34
99.20
98.93
98.18
98.26
50.61
61.37
45.52
48.83
51.58
46.06
45.63
51.85
48.04
47.48
47.28
63.40
46.35
52.27
50.17
553
1629
1671
1252
1623
1858
1360
1513
1614
1585
46.99
1573
63.82
1561
885
1103
1641
1561
1693
336
0.23
240
0.19
416
189
377
432
369
374
345
0.20
360
0.26
0.18
0.22
0.24
0.21
0.22
346
0.24
27
0.05
40
0.08
350
0.20
319
0.24
0.09
0.11
0.09
0.09
0.09
0.30
0.09
350
0.22
0.11
0.10
0.10
0.09
0.10
0.10
0.09
0.09
0.12
Label
115-4-6
115-4-7
113-4-8
113-4-9
R28-5-1
R28-5-2
1428-5-3
1128-3-4
1128-3-5
828-5-6
1128-3-7
1628-3-8
R2859 1128.3.10
Host
plag
plag
plag
plag
plag
plag
85
1220
plag
plag
plag
plag
plag
plag
plag
85
85
85
85
85
1220
1220
1220
1220
1220
85
1220
1220
30
25
45
20
40
15
10
49.95
50.36
50.02
49.84
49.32
49.06
1.05
1.03
50.32
0.97
0.71
0.80
16.12
16.17
16.32
17.83
15.56
Fo/An#
Temp
Size
85
1250
1250
1250
1250
1220
38
38
10
12
30
49.95
49.85
49.65
48.71
49.79
30
50.47
0.03
plag
0.21
0.02
0.76
0.70
1.01
0.66
0.75
0.75
0.94
0.87
49.87
0.94
17.45
17.36
20.07
20.25
16,04
16.10
16.12
16.25
16.14
Fe06
MnO
MgO
8.14
8.61
7.48
7.46
8.09
8.25
8.20
8.20
8,45
8.89
9.30
8.94
7.20
7.94
8.94
8.85
8.30
8.72
8.18
0.13
0.13
0.11
0.14
0.22
0.20
0.20
0.22
0.20
0.18
0.20
0.17
0.13
0.16
0.16
0.18
0.21
0.15
0.13
8.97
9.15
6.11
8,12
9.12
9.01
9.15
8.89
9.06
8.34
8.30
8.24
7.90
8.63
8.95
9.08
9.16
9.17
9.81
9.74
9.61
8.88
9.16
8.01
CaO
13.25
13.48
13.58
14.05
11.77
11.75
11.91
11.57
11.63
12.99
13.03
12.87
12.71
12.26
12.85
12.99
12,94
12.82
12.74
12.57
12.66
12.86
12.84
13.64
Na20
2.11
2,03
2.35
1.57
2.59
2.62
2.68
2.54
2.64
2.43
2.51
2,62
2.43
2.46
2.20
2.37
2.47
2.87
2.48
1.89
0.03
0.51
0.02
0.05
0.05
0.05
0.05
0.06
0.06
0.05
0.06
0.06
0.05
0.05
0.05
0.07
0.05
0.05
0.06
0.05
0.20
0.06
0,06
0.05
0.07
0.08
0.10
0.08
0.07
0.05
0.06
0.07
0.04
0.00
0.05
0.04
0.06
0.03
0.04
0.06
0.06
0.06
0.05
0.05
0.04
0.04
0.05
0.05
0.05
0.04
0.05
0.05
0.06
0.00
0.05
101.55
101.29 101.17
99.32
99.20
99.16
99.16 100.35 100.70 100.45
0.06
98.52
0.06
101.01
0.06
98.53
0.00
0.05
0.01
0,06
0.06
0.06
0.04
0.02
0.05
P205
Cr203
Total
Mg#
Sppm
Cl ppm
Ct/K
P/Ti
0.06
0.07
2.52
0.05
2.53
0.05
2.64
0.05
2,54
K20
97.32 100.05 100.05 100.80
99.96 100.03
99.76
99.49 100.18
99.78 101.19
66.25
65.47
59.28
66.00
66.79
66.06
66.54
65.92
65.66
62.58
61.40
62.16
66.17
65.95
64.09
64.64
66.30
65.20
68.12
67.85
68.00
65.30
66.05
65.88
681
970
525
756
1038
1058
1070
1038
1093
1189
1310
1192
884
1167
421
484
407
1021
1090
1046
1109
1039
1039
813
158
89
1429
81
62
48
54
50
60
119
99
85
52
73
43
41
52
88
70
78
81
147
93
107
0.36
0.36
0.34
0.59
0.16
0.12
0.13
0.15
0.24
0.20
0.17
0.16
0.17
0.10
0.09
0.10
0.22
0.16
0.26
0.24
0.27
0.07
0.20
0.09
0.07
0.07
0.07
0.09
0.09
0.07
0.07
0.03
0.08
0.07
0.08
0.07
0.05
0.01
0.18
0.01
0.18
0.08
0.14
0.07
0.02
0.08
0.07
0.01
Si02
Ti02
Al203
-
Table 4 (cont'd)
Label
P1155-8-23
P1165-I-I P8193-3-I
P1193-3-6 P11113-4-I P11113-4-2 P11113-4-3 P11113-4-4 P11113-4-S PHII3-4-6 P11113-4-i P11113-4-S PHII3-4-911113-4-19111I3-4-II'11ll3-4-1211113-4-13H113-4-1411113-4-1511113-4-16
plag
plag
ol
plag
plag
66
70
84
79
86
1160
1135
1135
1135
1210
70
1220
1230
1170
1220
1220
1220
15
37
17
10
7
10
16
10
14
30
50.38
50.37
50.49
50.45
50.99
50.00
48.96
51.24
50.25
20
49,34
51.15
ol
plag
Fo/An#
Temp
79
78
1170
35
48.94
Si02
P1193-3-4 P1193-3-5
plag
70
ol
Size
P1193-3-2 P1193-3-3
plag
70
Host
plag
plag
ol
ol
ol
85
85
82
1220
1210
1210
1210
30
22
30
Il
50.03
50.68
50.64
49.60
plag
ol
ol
ol
ol
ol
ol
ol
84
85
83
85
84
85
85
1210
1210
1210
1210
1210
1210
1210
4
45.85
15
8
Ii
1210
7
35
50.14
50.01
49.43
49.92
30
48.67
48.46
ol
51.53
24
2.22
2.63
2.90
2.39
2.46
1.94
1.41
1.82
1.54
1.09
1.22
1.18
1.36
1.47
1.46
1.24
0.62
3.63
1.67
1.52
1.49
1.65
1.54
1.38
Al203
13.49
13.22
13.74
14.17
13.59
16.41
16.36
13.98
12.29
15.18
15,27
15.43
15.07
14.59
14.99
15.80
15.28
11.87
13.97
14.08
13.93
14.10
14.12
14.31
Fe06
MnO
12.03
13.89
11.94
11.65
12.24
10.95
9.72
9.21
8.79
10.25
10.21
10.41
8.30
12.34
10.06
11.07
10.73
10.31
0.17
0.16
0.19
0.19
0.17
0.21
0.17
8.95
8.92
7.39
7.37
8.13
7.36
8.21
8.09
9.11
0.19
8.52
0.20
8.58
0.17
7.15
0.23
10.45
0.15
4.66
0.19
8.23
0.17
6.55
0.20
5.40
0.16
Mg0
0,24
5.30
8.66
8.40
9.76
0.16
8.47
10.01
0.24
5.52
7.59
0.18
8.70
0.22
5.98
9.06
0.20
11.14
0.18
10,31
10.05
10.03
9.94
10.05
10.98
12.48
11.47
12.01
12.36
12.70
12.04
11.87
11.96
11.86
11.42
12.02
12.42
11.91
11.66
11.87
11.49
11.68
11.72
2.74
3.14
3.45
3.54
3.20
2.93
2.74
1.90
2.50
2.88
2.60
2,81
2.54
2.45
2,63
2.85
1.54
2.56
2.53
2.44
2.76
2.54
2.43
K20
0.16
0.20
0.76
0.89
3.09
0.78
0.69
0.18
0.27
0.11
0.10
0.13
0.14
0.33
0.14
0.14
0.12
0.13
0.12
0.27
0.39
0.28
0.25
0.11
0.10
0.12
0.12
0.15
0.14
0.11
0.02
0.17
0.01
0.01
0.00
0.04
0.05
0.05
0.02
0.05
0.04
0.05
100.23
43.12
59.91
59.21
97.39
60.74
97.27
59.88
1779
757
928
1259
1187
1205
1439
58.86
653
99.16
58.99
1502
62.07
308
99.53 100.14
60.16 58.00
99.27
43.43
99.92
58.44
99.66
49.25
98.55
44.01
0.04
98.62
0.04
98.89
44.75
0.02
98.18
0.01
99.33
45.15
0.05
98.57
0.10
0.14
0.05
9999
0.02
98.07
0.14
0.05
0.13
0.00
0.14
0.04
0.13
0.02
97.02
0.28
0.00
0.09
0.10
0.23
0.20
0.10
0.13
0.18
P205
Cr203
Total
Mg#
Spprn
Cl ppm
0.19
0.18
1315
332
468
569
528
622
414
252
227
153
126
20
Cl/K
0.25
0.28
0.09
0.07
0.10
0.17
0.09
0.10
0.13
0.12
0.11
0.08
0.14
0.10
0.18
0.09
0.08
P/Ti
0.07
0.13
R28-i-Il
R28-7-12
1121-7-13
R28-7-14
R28-7-15
R28-7-16
1132-3-I
1132-5-2
plag
plag
plag
plag
plag
ol
ol
Ti02
Ca0
Na20
Label
Host
Fo/An#
Temp
plag
58.87
0.15
8.38
97.73 100.34 100.41
99.78 100.21
65.62
803
60.12
58.89
58.91
58.07
66.16
108
1247
1216
1055
1155
0.46
0.06
100.32
60.13
660
1295
1467
1323
1369
1230
55
141
176
185
159
10656
282
273
201
200
196
173
0.01
124
0.17
0.03
0.15
0.22
0.17
0.14
3.90
0.24
0.24
0.21
0,19
0.19
0.21
0.09
0.10
0.09
0.09
0.10
0.10
0.09
0.04
0.13
0.10
0.09
0.09
0.08
0.08
0.10
1132-5-3
1132.5-4
R32-5-5
1132-5-6
1132-5-i
1532-3-I
1932-5-9
1832-9-Is
1132-5-Il
1932-3-12
1532-5-13
1132-5.14
1132-5-IS
1932-3-16
1432-5-li
1132.5.18
ol
01
01
ol
ol
ol
ol
ol
ol
ol
ol
ol
at
ol
ol
88
88
ol
88
88
88
88
88
88
88
88
87
88
88
88
88
88
1260
1260
1260
1260
1260
1260
1260
1260
1260
1260
1270
7
32
10
7
10
5
50
25
47.86
48.27
47.94
48.21
49.02
48.67
49.37
49.03
20
49.13
90
48.27
210
0.05
1230
1230
1230
1230
1230
1230
1260
1260
1260
1260
1260
1260
1260
20
90
24
30
20
6
20
46
30
46
16
6
Si02
50.78
49.63
49.50
49.94
50.00
50.58
26
49.28
48.96
50.01
48.54
49.36
49.53
48.22
Ti02
Al203
0.39
0.83
0.83
1.18
1.16
0.89
1.01
1.00
1.01
1.08
1.17
1.02
0.52
1.02
1.50
1.02
0.90
0.91
0.97
0.95
0.93
1.01
1.02
0.98
16.82
16.88
16.86
16.62
16.57
16.99
15.00
14.97
15.62
15.64
15.36
15.52
18.83
15.64
15.45
15.01
15.36
15.33
15.05
14.56
15.15
15.34
15.23
14.48
Fe04
MnO
8.33
8.13
7.95
7.91
9.48
9.61
8.49
9.28
9.05
8.94
7.84
8.78
9.47
10.63
8.75
9,07
9.42
8.93
0.15
0,15
0.21
0.16
0.14
0.14
0.17
0.16
0.14
0.19
0.17
0.14
9.72
9.50
8.76
8.61
10.74
10.42
10.38
10.47
10.94
10,94
10.17
10.76
10.44
10.47
10.81
10.65
0.15
10.68
0.18
9.14
0.15
10.82
0.13
Mg0
0,22
10.72
9.47
0.17
10.35
0.17
8.68
0.15
9.58
0.19
7.99
0.14
7.86
0.16
10.85
10.62
0.17
12.04
CaO
11.97
11.45
11.56
11.61
11.30
12.19
12.21
12.17
11.74
10.38
11.29
10.56
10.74
11.00
11.39
11.46
11.67
11.48
10.70
2.42
0.05
2.35
2.42
2.45
2.27
2.31
2.38
1.99
2.30
2.65
2.22
2.48
2.35
2.18
2.24
2.22
2.21
2.10
0.05
0.06
0.17
0.05
0.13
0.05
0.02
0.06
0.38
0.04
0.06
0.05
0.10
0.07
0.09
0.09
0.10
0.11
0.09
0.04
0.10
0.25
0.08
0.10
0.08
0.08
0.11
0.23
0.07
0.01
0.06
0.28
0.06
0.08
0.20
0.11
0.04
0.06
0.08
2.24
0.05
0.05
0.04
0.05
0.04
0.06
0.05
0.05
0.06
0.06
0.06
0.05
0.05
0.07
97.96
66.33
99.90
65.04
99.57
99.08
99.80
98.06
Size
49.63
12.69
12.73
13.04
12.22
8.86
11.94
Na20
1.88
2.36
2.24
2.46
2.26
K20
0.06
0.06
0.07
0.06
P205
Cr203
Total
Mg#
Sppm
0.01
0.07
0.09
0.09
0.06
0.09
0.06
0.05
0.05
0.04
0.05
0.05
0.07
100,32 100.64
100.30
99.43
99.11
99.85
99.79 100.14
98.96 100.63 100.86 100.79 100.42
98.55
97.81
66.15
68.04
68.04
66.37
66.41
99.62
66.13
66.83
66.57
68.64
66.59
67.33
68.31
71.32
69.20
65,41
68.60
97.58
66.27
64.10
68.51
68.08
99.69
66.77
988
856
514
1049
1096
997
1155
1062
1010
1241
1106
988
373
1107
1158
1241
998
1022
1215
1241
836
741
907
1003
84
82
285
87
100
17
13
13
26
17
16
13
17
110
23
17
19
III
33
0.17
0,50
0.19
0.23
0.04
0.03
0.01
0.07
0.04
0.07
0.03
0.06
0.08
0.13
0.06
0.45
0.07
0.08
0_OS
0.09
0.09
0.10
0.10
0.09
0.08
0.17
0.08
0.04
0.08
0.04
0.10
0.03
0.10
29
0.05
0.08
0.02
0.24
32
0,07
35
0.17
0.02
79
0.17
0.11
0.08
0.08
0.12
0.23
0.20
0.07
Clppm
Cl/K
P/Ti
0.07
70.62
427
Table 4 (cont'd)
Label
Host
Fo/An#
Temp
Size
5i02
Ti02
Al203
PeOt
MoO
MgO
CaO
Na20
K20
P205
Cr203
Total
Mg#
1113.4.17H113-4.I811113-4-19Htt3'4'20H113-4'2t I1II3-4-22H1t3.4.23'11t13-4'24
ol
85
84
1210
1210
50
1210
42
25
1210
20
49.83
1.63
14.23
10.17
0.15
8.46
50.39
50.23
50.03
1.43
14,63
1.43
14.53
1.34
14.58
1306
182
10.46
0.19
8,63
11.47
2.38
0.10
0.12
0.05
99.72
59.52
1179
9.64
0.16
8.48
59,72
10.04
0.15
8.70
12.13
2.50
0.11
0.14
0,05
100.42
60,69
0.04
98.75
61.04
1260
158
110
0.20
0.10
0.20
0.09
032.519 032.5.20 R32'5.2I
15
5032
1.34
14.27
11.10
0.14
8.49
11.88
2.45
0.11
0.13
0.04
100.42
57.70
11.84
2.61
0.14
0.15
0.06
99.41
Sppm
Clppm
1277
174
1283
188
Cl/K
P/Ti
0.19
0.10
0.17
0.09
Labd
ol
85
ol
Fo/An#
Temp
Size
88
1270
ol
88
1270
110
35
85
5i02
Ti02
Al203
Fe00
49.19
49.67
1-lost
MaO
MgO
CaO
Na20
K20
P205
Cr203
Total
Mg#
Sppm
CI ppm
CE/K
P/Ti
ol
1.03
48.62
0.99
14.62
9.08
15.08
9.18
0.46
0.19
11.22
ol
88
1270
931
69
1181
1,04
15.16
8.63
0.41
11.46
11,18
2.26
0.05
0.07
0.06
99.80
70.30
942
13
l3
0.12
0.12
0.04
0.13
0.03
0,07
11.82
11.54
2.11
0.07
0.12
0.06
99.90
69.88
11.66
2.31
0.04
0.13
0.06
99,61
68.53
ol
ol
85
1210
85
ol
83
1210
20
ol
ol
85
1210
85
1210
R3-2-t
R3-2-2
R3-2-3
R3-2-4
R3.2.5
R32-6
R3.2-7
R3-2-8
R3-2-9
R3-2-tO
R3'4-t
tt3-4.2
R3-4-3
R3-4-4
R34-5
plag
88
1210
plag
plag
plag
88
plag
plag
89
1220
40
plag
plag
plag
plag
plag
plag
89
1210
plag
89
1220
plag
89
1210
plag
89
1220
88
1220
1230
1230
1230
1245
18
80
50.64
0.8I
48.76
0.88
51.11
50
50.47
0.89
1230
37
51.04
0.97
50.88
1,04
16.19
7.73
0.19
9.46
12.81
2.20
0.06
0.06
0.05
15.42
8.18
0.13
10.20
12.38
2.17
0.02
0.07
0.06
51.83
0.88
15.42
9.01
0.19
9.75
12.93
1.67
0.03
0.07
0.04
89
1220
1220
20
25
10
25
5
18
6
12
40
49.44
1.64
14,26
49.70
48.90
1.45
14.64
50.26
0.74
10.67
0.17
8.46
11.50
7.41
0.17
8.72
13.31
2.11
0.03
48.53
0.85
15,55
8.23
0.17
10.06
12.36
2.15
0.03
0.04
0.07
0.14
0.09
2,53
0.10
0.13
0.05
99.53
58.56
1186
232
0.29
0.09
50.59
0.90
16.12
8.13
0.18
9.76
12.96
1.95
0.08
0.05
0.04
49.90
0.81
17.82
10.76
0.19
8.35
10.85
2.58
0.10
0.10
0.04
98.42
58.03
1300
167
0.19
0.06
50.06
0.92
15.40
8.80
0.18
9.61
13.03
2.08
0.05
0.07
0.06
49.49
0.95
14.67
50.78
0.86
15.60
8.59
0.20
8.98
13.26
032.5.22
032.5.23
032.5.24
032.9.25
034.3.1
R34.3.2
034.3.3
ol
88
1270
20
48,93
ol
ol
ol
ol
88
1260
81
ol
88
1270
25
1200
80
ol
82
1200
12
49.72
50.28
1.02
15.98
9.00
0,14
10.60
11.22
2.51
0.06
0.09
0.04
1.82
13.81
0.99
15.15
9,20
0.14
11.11
11.19
2.08
0.05
0.08
0.05
99.08
68.28
980
24
0.06
0.08
11.68
2.44
0.10
0.13
20
48.13
lii
15.24
9.94
0.18
10.70
11.62
2.23
0.15
0.12
0.06
99.66
1260
15
48.79
0.99
15.30
9.63
0.16
10,50
10.66
2.35
0.07
0.10
0.04
98.72 100.49
65.74
66.02
67.74
1216
473
0.37
0.11
1162
54
1067
0.09
0.10
0.03
0.09
13
1.39
11,90
0.19
7.92
11.32
2.66
0.13
0,18
0.02
100,22
54.27
1540
94
0.09
0.10
1.79
14.10
11.35
0.21
7.71
11.29
2.72
0.14
0.16
0.01
99.88
54.79
1483
122
0.11
0.09
50.46
1.76
13.70
11.83
0.25
7.88
11.11
7.98
16.28
44
50.12
0.94
15.83
8.03
0.16
10.19
12.63
2.05
0.05
0.06
0.05
85
35
0.83
15.57
9.00
0.16
10.32
13.68
1.42
0.04
0.07
0.07
68.58
434
67.72
1060
7.79
0.16
10.01
12.40
2.04
0.03
0.04
0.05
99.80
69.59
990
57
130
112
119
173
0.04
0.08
0.49
0.05
0,43
0.05
0.55
0.05
0,46
0.06
68.55
946
244
0,48
0.07
R34.3'4
534.3.5
1334.3.6
1334.3.7
1334.3.9
1134.3.10
1134.3.11
ol
ol
ol
ol
ol
81
ol
84
1200
ol
81
ol
84
1200
84
1200
85
1200
81
81
1200
8
23
5
8
1200
6
1200
10
49.73
49.46
49.69
50.32
49,06
1.81
1.72
13,55
10.52
0.12
7.96
11.50
2.63
0.12
0.16
0.08
98.32
57.41
1410
106
0.11
0.09
13,61
1.75
13,44
40.59
49.43
1.72
13.55
12.21
13.45
42.47
0.21
7.81
11.01
0.19
8.29
10.86
2.66
0.12
0.17
0.02
50.43
1.90
13.80
10.76
0.24
8.08
12.00
2.69
0.12
0.18
0.09
10.60
0.18
8,28
11.14
1.87
2.58
0.03
0,10
0.10
0.08
0.05
0.07
98.12 100.31 100.26 100.77
58,19 65.08 66.06 68,14
532
389
276
1262
217
71
112
77
0,11
0.35
0.56
0.13
0.07
0.09
0.08
0.06
1200
40
50.41
15.67
89
1220
1200
10
50,47
1.70
13.90
11.89
0,21
8.01
11.37
2.83
2,65
0.13
0.13
0.16
0.18
0.02
0.01
100.31 100.63
54.29 54.56
1258
1435
108
8l
0.10
0.10
0.08
0.10
0.21
9.77
12.74
2.51
0.18
0.07
0.05
0.04
0.05
99.62 100.37
1200
45
49.73
1.77
13.59
12.08
0,19
8.06
11.08
2.91
0.14
0.17
0.02
99.91
54.30
1518
146
0.13
0.10
40.51
0.18
8.11
11.80
2.62
0.12
0.13
0.07
98.48
57.89
1464
107
0.11
0.08
16.00
9.39
0.15
9.59
13.53
1.99
0.03
0.06
0.05
16.11
8.91
0.16
9.51
13.51
2.10
0.03
0.07
0.06
15.81
9.21
0.16
8.95
12.95
2.40
0,12
0.09
0.01
98.04 100.10 100.20
98.26 102.36 102.26 102.57 101.95 101.66
68.55
1118
69,34
68.96
1179
1334.3.
981
1.61
115
0.57
0.08
1.71
13,39
10,28
0.16
8.01
11.34
2.73
0.12
0.17
0.06
98.34
58,15
99.07
53.26
1376
98
1415
14.44
1288
100
107
94
0.09
0.09
0.10
0.10
0.10
0.09
0.09
0.10
0.19
7.92
11.49
2.64
0.13
0.16
0.07
98.20
57.13
2.67
0.13
0.16
0.01
65.86
63.41
1093
110
181
0.33
0.07
65.55
1065
99
0.42
0.07
0.42
0.07
0.02
0.09
R343'I2 034.3.13
034.3.14
R34'3'15
034.3.16
R34.3.17
ol
ol
ol
67.15
960
43
0.12
0.08
px
12
99.15 100,47
54.24
57.23
1550
124
0.12
0.09
64.55
1068
87
19
ol
83
1200
81
81
81
1200
1200
1200
ol
88
1200
20
50.24
24
48.62
35
62
50.12
49.04
1.30
14.36
1.74
13.42
12.66
0.18
7.63
10.46
2.57
0.14
1.81
13.67
13.78
10.95
0.18
8.04
12.20
2.47
0.09
0.10
0.04
100.08
56.70
1220
76
0.10
0.07
0.16
0.02
97.78
51.78
1541
181
0.16
0.09
48.71
1.74
13.43
12.29
0.21
7.63
10.48
2.71
0.14
0.18
0.02
97.70
52.53
12.20
0.21
8.14
10.58
2,68
0.14
0.17
0.01
1377
121
99.90
54,33
1450
136
0.11
0.11
0.10
0.10
10
1.71
12.09
0.20
7.60
10.35
2.73
0,12
0.19
0.02
97.99
52,83
1515
140
0.14
0.11
Table 5.
Label
Ti
Sr
Y
Zr
Ba
La
Ce
Sm
PH55-8-5 PH55-8-6 PH55-8f8 P1155-8-2t PH55-8-25 PH93-3-1 PHII3-4-4 PFIII3-4-7 PH113-4-14 PHhl3-4-I
12990
1502
563
768
23.8
229.9
260.0
Eu
66.3
204.1
Dy
100.0
Er
Yb
121.9
107,8
(LaISm)
3.47
Ti/Zr
(Ce/Yb)
16,91
Label
2.41
15291
120
50
155
19.2
10001
21.5
30.0
40.5
42.7
13.8
79
31
89
10.1
16.9
21.5
14257
120
46
140
18.5
11381
21.9
26.5
32.6
32.3
32.0
31.2
27.8
0.67
12.8
76
30
93
10.8
16.0
22.2
20.6
22.5
22.5
39.2
21.9
20.2
22.1
35.2
18.8
18.3
36.8
0.58
0.53
0.64
98.53 111.86 101.76 122.19
0.95
0.85
0.81
0.92
18577
258
37
6182
7884
9859
8824
106
16
91
121
113
22
33
195
43
4.1
5,2
61
5.1
92
9.2
7.7
13.6
24
79
8.6
11.3
116.4
65.3
63.0
39.9
64.6
27.3
23.9
22.6
7.3
10.5
17.5
11.8
11.7
14.3
14.0
13.0
14.8
26.3
25.3
11.1
14.5
17.6
11.0
14.5
25.7
22.6
10.3
15.3
20.1
15,7
0,54
0.52
0.44
4140
5127
6688
3548
3438
5294
6504
69
129
143
117
120
131
14
14
18
23
14
13
113
19
26
1.0
3.0
24
0.8
2.8
47
61
1.9
31
27
45
1,3
2.0
1.6
8.2
3.1
3.5
Eu
Dy
Er
Yb
(LaJSm).
Ti/Zr
(Ce/Yb)
Zr
Ba
La
Ce
Sin,
6182
4760
5732
107
73
18
14.2
28
76
7.5
11,3
82
23
30
0.8
3.2
19.9
34.4
14.2
23.2
4.9
6.8
13.8
16.1
29.3
28.8
25.9
22.5
0.41
94,58
0.88
22.0
19.2
14.3
18,8
12.9
17.1
14.1
0.49
0.23
16.8
20.7
19.5
19.7
0.26
9804
140
33
104
12.7
13.8
45
5055
84
19
36
5288
82
20
35
1.4
7407
107
26
50
1.3
1.7
4.2
3.8
6.0
16.9
17.7
21,3
4.8
5.5
10.9
13.4
18.4
19.9
14.1
15.5
22.0
0,23
13.5
13.2
0.28
1.6
5.2
7.1
18.3
13.8
0.44
81,59 159.15 127.90 141.49 152.03 147.80
0.83
0.35
0,34
0,27
0.41
0.53
R3-4-5 R3-4-6 R3-4-7 R28-3-5 R28-3-7 R28-7-8 R28-7-9 R28-7-12 R32-5-3 R32-5-4 R32-5-5 R32-5-1O R32-5-15 R34-3-1 R34-3-2 R34-3-5 R34-3-6
4254
74
Y
16.5
0.80
95.40 143.95 128,57 106.59 111.35
0.87
0.75
2.79
0.71
0.68
1.64
6494
88
25
47
8.6
6.5
7.8
17.0
19.0
Ti
Sr
18.8
P11113-4-18 P11113-4-19 R3-2-7 R3-2-8 R3-2-9 R3-4-2 R3-4-3
1,9
23
59
11.5
10.3
6320
94
19
54
1.6
8379
96
25
60
9,0
9.6
8.8
5811
73
5664
19
18
42
0.3
4.4
42
6.4
5.6
85
11101
132
39
114
10.6
17.5
10.2
5.0
5.1
5.7
8.0
12.8
5.3
7.6
6.6
6.2
21.7
14.6
13.7
13.1
10.5
19.0
15,9
19.0
14.2
11,1
4,2
4.4
9.3
10.3
6.9
9.8
17.0
11.7
11.6
15.6
18.4
15.6
15.7
12.7
18.2
13.9
17.3
13.7
13.3
29.4
28.3
20.4
11,0
11.1
15.2
15.4
19.2
19,4
14.6
17.3
13.5
18,8
15.2
12.1
16.8
10.7
10.7
12.8
16.2
19.1
18.7
12.3
16.6
13.1
15.9
14,2
11.1
17.9
9.0
0.32
9.1
14,3
15.7
18.6
18.6
13.6
13.7
11.2
14.1
12.4
0,28
0.40
0.56
0.22
0.26
0,54
0.55
0.33
0.50
0.31
0.38
137.84 165.19 170.21 109.80 108.94 112.86 125.51 118.49 109.83 117,72 139,47 139.18
0,43
0.47
0.49
0.69
0.65
0.27
0.28
0.59
0.93
0.68
0.63
0.53
11020
134
9082
10774
105
124
37
110
30
38
87
110
8.8
17.2
7.9
8.6
12.2
14.9
15.9
19.7
21.5
19.6
23.6
27.4
28.5
21.7
25,8
30.3
30.0
20.1
26.9
27,1
19.8
11.5
25.8
28.5
17.7
0.50
2.12
0,54
0.59
0.67
0.57
29.7
26.6
28.6
0.63
98.32
0.69
96.98 100.28 104.22
0,84
0,76
0,90
-
75
Compared to other melt inclusion populations, the EPR melt inclusion
compositions show little variation within a host lava, and even less diversity within a
single crystal (Fig 1 2b). In this case, we refer to diversity () as the chemical variation of
an element in melt inclusions from a single host lava. The range of MgO (MgO) in
inclusions from within a single sample is typically <2 wt %, but can range up to 4.3 wt
% MgO, as in the case of R3-4 (Fig 12b). Within a single crystal, AMgO varies from>
0.1 wt % MgO to - 1.3 wt %. These variations are generally similar to the ranges
exhibited by the lava suite, but differ from other regions (e.g. Endeavour Segment of
JDF) where, within a single sample, inclusion diversity exceeds that of the lava suite,
with zMgO>4%.
Although the range in melt inclusion MgO within a single host lava is relatively
small, incompatible elements such as K20, P205 and Cl commonly vary by a factor of 2
to 40 (Fig 13, 14). Nonetheless, at a given MgO, the EPR melt inclusion variations are
less than those reported for other melt inclusion suites, but greater than the variation
within the host lava suites and larger than can be attributed to analytical error. Like other
suites, however, the incompatible elements are strongly correlated. As a single group, the
melt inclusions exhibit positive correlations between the rare earth elements (REE), Zr,
Y, Ba and K20, Ti02, and P205 (Fig. 15), but a negative correlation with MgO, and no
correlation with Sr. In addition, K20 correlates positively with La/Sm and negatively
with Ti/Zr (Fig 16). N-MORB inclusions from axial samples have La/Sm ranging from
0.4 1-0.80 and Ti/Zr from 95-144. Seamount inclusions have lower La/Sm and higher
Ti/Zr, ranging from 0.22-0.67 and 102-170, respectively. These ranges of Ti/Zr and
La/Sm are not significantly greater than analytical error.
Relationship Between Melt Inclusions and Their Crystal Hosts
Of the ten lava samples used in this study, four have only plagioclase phenocrysts,
three have only olivine phenocrysts, and two have both olivine and plagioclase, and one
has olivine and clinopyroxene. As a group, the MgO content of the
76
Fig. 14 Electron microprobe analyses of minor and trace elements of melt inclusions
from axial and seamount lavas of the northern East Pacific Rise. Samples and symbols
are the same as in Fig. 12 and 13. Note that Cl and K20 inclusion populations of both
axial and seamount lava groups are similar and both form near-vertical assimilation-type
trends. Host lava concentrations were not available for these elements. The distinct
PHi 13-4 inclusion described in the caption to Fig 12 is indicated similarly here.
Fig. 15 La and Ba versus K20 and Ti02 for axial and seamount melt inclusions. The
three fields represent the three different lava suites: 1 O°30'N (striped), 11 °20'N (light
gray), and seamounts (dark gray). Samples and symbols are as in Fig. 12, 13, and 14,
with the addition of axial sample P93-3 and seamount samples R28-3 and R28-7. Note
that the axial and seamount melt inclusion data overlap and behave similarly, however
the most depleted compositions are represented by seamounts. As a group, the melt
inclusions show more diversity than either of the axial lava suites. The one E-MORB
melt inclusion from sample PH93-3 is distinct from the N-MORB lavas and inclusions.
Fig. 16 Ti/Zr and LaISm versus MgO and K20 for axial and seamount melt inclusions.
Samples and symbols are the same as in Fig 15. Note the narrow range of values in the
inclusions relative to the host lava suite and that the E-MORB inclusion falls away from
the N-MORB lavas and inclusions.
Axial
0.6
Seamount
0.6
0.5
0.5
0.4
0.4
o6
0
0
x
X
O
03
0.3
0.2
0.2
+.f
A
0.0
I
6
4
600
I
A
A
500E400
44.
i
0
0.2
I
I
oo
100)
0.8
1
X
0
0
0.6
K20
R3-4M1
XR32-SotMI
R34-3oIMI
X
1400
I
0.4
12
o R3-2M1
-
500-
A
+PH55-8 ol MI
PH93-3 ol Ml
APH93-3 fsp MI
OPH113-4 ol MI
PH113-4 fsp Ml
2O0
10
MgO
600
A
0
8
700
-
.
6
4
MgO
100-
0
-
0.0
I
12
10
8
700
x
0.1
I
0
0.2
0.6
0.4
K20
0.8
1
Lfl
1
12
220
200
180
0.9
160
140
0.6
120
0.4
100
0.2
80
60
4
8
9
II
10
12
13
MgO
0
4
9
8
10
12
MgO
13
1.2
220
7
(876, 1.6$) A
200
ISO
0.8
160
0.6
140
120
0,4
108
60
0.00
0.05
0.10
6,15
0.20
K20
0.25
0.30
0.35
0.00
0R3-2 p1
2 1O°30'N Lavas
R3-4 p1
II Searnount Lavas 0R28-3 p1
+P55-8 ol
R28-7 p1
A P93-3 P'
XR32-5 ol
0P113-4 ol + Ipi R34-3 ol
0.2
80
D11°20N Lavas
0.05
0,10
0.15
0,20
0.25
0.30
0.35
80
melt inclusions is positively correlated with the forsterite and anorthite content of the
inclusion host crystals (Fig 12a). Both olivine and plagioclase host a range of melt
compositions, but olivine holds the most primitive inclusions, and plagioclase the most
evolved. This is opposite to the relationships noted for the plagioclase ultraphyric lavas
from the Galapagos Platform, the Gorda Ridge and the Endeavour Segment of the JDF
where, in almost all cases, the most primitive phenocrysts were plagioclase, and olivine
was a minor, relatively evolved phase. For those samples that have both olivine and
plagioclase phenocrysts, melt inclusion compositions overlap one another, but the
olivine-hosted inclusions are generally more primitive (Fig 12). In addition, for all but
one sample, for which we have oniy one datum, the olivine-hosted inclusions are more
magnesian than their host lava. In contrast, the feldspar-hosted inclusions exhibit
compositions both more and less magnesian than their host lavas.
In 26 of the 100 crystals used in this study, we were able to analyze as many as 5
inclusions >5 from the same crystal. Within the olivine phenocrysts, multiple inclusions
from the same crystal have nearly identical compositions. Within the feldspar, on the
other hand, the melt inclusion compositions are more variable, with MgO> 1 wt % at
high anorthite contents. At low anorthite contents, there is little variation in the MgO of
the inclusions.
To summarize, the seamount lavas differ from axial lavas in that they are more
crystal rich and they contain a greater number of inclusions that are generally more
primitive and they exhibit a larger compositional range in both the incompatible and trace
elements.
81
Discussion
Global Framework
Previous studies (Sours-Page et al. 2000; Nielsen et a! 1 995a) have found that
melt inclusions are genetically related to their host lava suite, but often exhibit greater
chemical variation at a smaller scale. In the same way that we observe a greater diversity
of lavas from slow and intermediate spreading ridges than from fast spreading ridges
(Sobolev 2000; Sobolev and Shimizu 1992), we might expect melt inclusions to be more
diverse at slower spreading ridges. This prediction is based on a number of factors, but
ultimately relates back to the temperature of the underlying mantle. At slow spreading
ridges, the mantle is often cooler, the magma flux is non-robust, and melts do not pooi in
any detectable melt lens before erupting. Therefore, the magmas have little opportunity
to mix, and one should see melt inclusions representing this heterogeneity. At fast
spreading ridges, the mantle is warmest and the magma supply is generally robust. These
conditions are sufficient to support a steady-state melt lens, which provides a place for
melts to pooi and homogenize before reaching the surface. Such environments are the
least likely to transport phenocrysts from deep in the crust, and therefore also the least
likely to provide an abundance of primitive melt inclusions. Those melt inclusions found
will have passed through or formed in the axial magma chamber, and may preserve little
of the original diversity of melts. Hence, the same rules apply to melt inclusions as do to
lavas: the slower the spreading rate, the more likely that the original melt diversity has
been preserved.
In the slow and intermediate spreading regions studied, including Juan de Fuca,
Southeast Indian, Mid-Atlantic, and Chile Ridges, as well as the Blanco Fracture Zone,
samples typically contain large, broken plagioclase crystals. Nielsen et al. (1 995a)
speculated that they are xenocrysts, formed on the walls of magma conduits and ripped
off with recharge injections. Such crystals generally contain tens to hundreds of
inclusions with a variety of compositions, which often have formed concentric bands
(Nielsen et al. 1995a). For example, the Endeavour Segment, noted for erupting N-
82
MORB and E-MORB lavas in close proximity, produces inclusions that exhibit extreme
compositional variations, ranging from ultradepleted N-MORB to E-MORB, often within
a single crystal (Sours-Page et al. 1999). Within the Endeavour lava suite, all of the NMORB host lavas contain both N-MORB and E-MORB inclusions, whereas the EMORB host lavas contain only E-MORB inclusions.
Unlike these regions, the fast spreading northern East Pacific Rise lavas contain
only a few small, euhedral crystals. It differs from the Endeavour Segment in its lack of
phyric lavas, the scarcity of inclusions, the lack of chemical variation among melt
inclusions from the same crystal, the lack of overall compositional variability, and the
absence of E-MORB inclusions in all but one of the N-MORB host lavas. The differences
between the EPR and other regions speak to a fundamentally different axial plumbing
system beneath the East Pacific Rise. However, like other regions, the EPR melt
inclusion compositions are very similar to the EPR lava compositions, suggesting that in
all cases, the two populations are genetically linked.
In addition, while each host lava contains a unique population of inclusions, as a
whole, the melt inclusions from the two axial segments are indistinguishable. Because
the melt inclusionlhost relationship does not vary as a function of latitude, and therefore,
spreading rate, the magmatic processes (i.e. evidence of a melt lens, presence of a crystal
mush zone, etc.) observed on single segments are most likely non-unique and may be
applied to other segments of the northern East Pacific Rise as well. Furthermore, the fact
that a melt lens or mush zone has not been detected everywhere is not evidence for its
absence from segment to segment, because the fundamental relationship between melt
inclusions and their host lavas is preserved irrespective of location and spreading rate. In
other words, although differences in magma flux arid robustness of segments have been
observed, they are apparently insignificant to the magma plumbing system, and cause
little, if any, change in the processes creating lavas in the areas studied here.
83
Differing Histories of Plagioclase and Olivine
On the northern EPR, olivine phenocrysts exhibit more primitive compositions,
on average, than the coexisting plagioclase, and they also contain more magnesian
inclusions. These two observations are consistent with previous studies of mineral
stability which suggest that olivine forms at greater depth, and therefore has access to
more primitive melts. While feldspar also exhibits primitive compositions and contains
some high magnesium inclusions, the plagioclase-hosted inclusions are generally more
evolved and therefore, must reflect a shallower, more recent part of the melt history. In
addition, olivine has a higher density than feldspar, and therefore, it is much more likely
to settle out, enhancing any existing differences due to the conditions of entrapment.
Magmatic Processes
Axial and seamount lavas from the N-EPR share a number of similarities that
suggest that they originated from the same mantle source. The comparable crystal
content and chemistry, melt inclusion diversity (MgO) and absolute range of
compositions all corroborate what others have suggested previously, that the axial and
near-ridge seamount magmas are formed in the same mantle melting regime, and diverge
at some point later (Batiza et al. 1990; Allan et al. 1989). This is confirmed by the trace
element data, wherein the most primitive compositions (least melted, least fractionated)
are represented by both axial and seamount inclusions (Fig 16).
If we assume that axial and seamount lavas share their source, it follows that all of
the lavas erupted from this area must be derived from a single array of parent magmas.
Diversity within that array could result from a number of factors, including the mantle
source heterogeneity, differing degrees of partial melting, and different types of melting
that might occur (e.g. batch versus fractional melting). Based on the fact that the
inclusions are not sufficiently primitive to be in equilibrium with the mantle (MgO -1 1.5
84
wt %, Mg #-75), it is assumed that the constituents of the original primary magma array
were not preserved as inclusions.
Relative to other regions, the N-EPR melt inclusions exhibit small variations
within a single sample. However, this diversity is greater than analytical error. Both
axial and seamount host lavas contain melt inclusions that are more diverse at high MgO
and narrow with decreasing MgO. In order for the inclusions to achieve and maintain
this diversity, the primary magmas must have undergone fractionation and not yet have
mixed. Assuming that mixing takes place in the melt lens, this would suggest that
fractionation is occurring before mixing, most likely in the magma conduits and mush
zone (Fig 13, 17). This pattern is similar to what has been observed in inclusions from the
Juan de Fuca Ridge (Fig 18). If the processes were reversed, one would expect to see
equivalent diversity at all MgO (Fig 17). For fractionation to occur outside the melt lens,
the magma must obtain its heat of fusion from some source other than magma
accumulation. In addition, the fact that seamount lavas are generally more primitive than
their axial counterparts suggests that, assuming the two lava groups are derived from a
similar array of parent magmas, the seamounts are leaving the axial plumbing system
from a deeper level. However, since there is greater variation at high MgO in both axial
and seamount lavas, we may presume that for both groups fractionation is taking place
prior to mixing in the melt lens.
The relationship between the host lava and its associated melt inclusions provides
some perspective about how and where the melt entrapment occurs. We have identified
three melt inclusion/host relationships (Fig 19). The first is where all of the inclusions
from a single host are more primitive than that host itself. In this case, the melt
inclusions were trapped as the melts differentiated, leaving snapshots of the intermediate
steps in the evolution of the lava. This type requires only one batch of melt which is then
fractionated to make the lava. Host lava R32-5 exhibits this behavior (Fig 13). The
second type of behavior, wherein all of the inclusions cluster around the host
composition, is attributable to melts of many different compositions combining to form
the host lava. This type requires melts to have pooled and mixed prior to entrapment, and
requires that all the melt inclusions approximate a single composition.
85
Fig. 17 Cartoon representing the different trends associated with mixing and
fractionation, depending on the order in which processes take place. When fractionation
precedes mixing, inclusion compositions become more diverse with decreasing MgO
until they are mixed. When mixing precedes fractionation, inclusion compositions reflect
very little diversity at any MgO.
Fig. 18 K20 versus Mg# for plagioclase-hosted melt inclusions in sample E-5 from the
Endeavour Segment of the Juan de Fuca Ridge (Sours-Page et al. 1999). Crosses
represent individual inclusions, while shaded areas represent the compositional extent of
two chemically distinct, yet coeval lava suites, the E-MORB (dark gray) and the NMORB (light Gray) from Karsten et al. (1990). Note the diversity of melt inclusion
compositions at both high and low MgO.
Fig. 19 Cartoon representing the three different host lava-melt inclusion relationships
observed in the N-EPR lavas. In (a), all of the melt inclusions are more primitive than
the host lava, and are presumed to represent intermediate melt compositions trapped
during differentiation. In (b), all of the melt inclusions cluster around the host lava,
suggesting that the melt inclusions were trapped in the melt lens where all of the magmas
had already pooled and mixed. In (c), the melt inclusions are of dramatically different
compositions (N- vs. E-MORB) and necessarily must represent genetically unrelated
origins. In this particular case, the crystals containing E-MORB inclusions are most
likely xenocrysts picked up by the magma during transport.
86
Figure 17
Fractionation, then Mixing
Mixing, then Fractionation
fractionation liquid line of descent
Mg#fMgO
Mg#/MgO
87
Figure 18
40
45
50
55
60
Mg#
65
70
75
80
Incompatible Element
C
Incompatible Element
Incompatible Element
89
Host lava R3-2 shows this behavior (Fig 13). The third type of behavior is more
complicated, and reflects E-MORB inclusions trapped in an N-MORB host. If we
assume that the few inclusions we have analyzed are representative of the entire inclusion
population, then it is clear that an aggregate of those melts would not form the host lava
composition. Instead, it is likely that the E-MORB inclusions were entrapped in crystals
formed in an E-MORB magma, which were separated from that original magma and
picked up as xenocrysts in the lava dredged at the surface. This hypothesis is supported
by the extreme differences in composition between the E-MORB inclusions and their
host, and the inability of any known chemical process to link one with the other. Host
lava PH93-3 exhibits this behavior (Fig 13).
Melting, fractionation and mixing can account for the chemical characteristics of
the both the axial and seamount lavas and melt inclusions erupted on the northern EPR.
However, based on the paucity of crystals within those lavas, magmas must undergo an
additional mechanical process that causes the separation of crystals from the erupting
magma. Crystal sorting could occur anywhere in the axial magma chamber. Crystals
could be filtered out within the mush zone or during entrance or exit from the axial melt
lens. In any case, because the seamount lavas are more crystal-rich and less
differentiated, we must assume that the axial lavas undergo more fractionation and crystal
loss than their seamount counterparts. One wat to account for the greater crystal sorting
and differentiation of the axial lavas is to have magmas formed in the melting regime
pass through a melt lens at the base of the crust, like that observed in Crawford et al.
(1999). The lens would act as a preliminary differentiation site and a filter to crystals.
Magmas would exit the lower melt lens with fewer crystals and lower MgO contents and
travel upward through the crystal mush zone until they are tapped by off-axis seamounts
or drawn into the upper melt lens.
To summarize, we believe that axial and seamount magmas are created from a single
parent magma array representing melting of a relatively homogeneous source. The
parent magmas undergo fractionation and mixing, during which time melt inclusions are
entrapped and phenocrysts separate from the magma (possibly in the lower melt lens). At
some level, the axial and seamount magma paths diverge. Axial lavas continue through
90
the axial magma chamber, becoming more fractionated and having more phenocrysts
removed (possibly in the upper melt lens). Seamount magmas travel to their volcanoes
without further significant fractionation and crystal separation (Fig 20).
Conclusions
On the northern East Pacific Rise, the diversity of melts created in the mantle
melting regime is difficult to ascertain from lavas erupted at the surface due to processing
that takes place in the axial magma chamber. To circumvent this problem, we have
obtained information on plagioclase and olivine-hosted melt inclusions. Our
interpretation of this data indicates that:
Based on the similar degree of diversity (MgO) and variability within each group,
we believe that axial and seamount lavas share a common mantle source. Because
there are systematic differences between the two magma groups in terms of degree of
fractionation and crystal content, their paths must diverge prior to their entrapment
and eruption. Seamount magmas form in the melting regime with axial lavas, but do
not pass through the crustal magma chamber, hence maintaining a more primitive
character; whereas axial lavas travel through the entire axial plumbing system
(melting regime, mush zone, and melt lens), attaining a homogeneous, aphyric
character.
Given the fact that both groups exhibit greater diversity of melt inclusions at high
MgO, fractionation preceded magma mixing in the development of both the axial and
seamount lavas in this region of the East Pacific Rise. This implies that much of the
heat generated by the crystallization of these magmas was emplaced in the lower and
middle crust and not transported to the melt lens, or to the surface.
Since the magmas show evidence of crystal fractionation, yet the erupted lavas are
aphyric, the axial melt lens must serve as a filter for lower crustal phenocrysts,
keeping them from traveling with the melts to the surface.
91
Fig. 20 Cartoon representing the axial plumbing system necessary to supply magmas to
both axial and seamount eruptions. Axial and seamount magmas are created in the
melting regime, pass through the lower melt lens, then diverge, allowing axial and some
seamount lavas to pass through the upper melt lens, thereby filtering out any preexisting
phenocrysts, and allowing the remaining seamount lavas to travel off-axis, retaining their
crystals.
92
Figure 20
Off-Axis
Volcanis
Axial Volcanism
'
Crystal Mush Zone
Moho
I
*
Melting Regime
93
Because inclusions from axial segments of differing robustness are indistinguishable,
the differences in magma flux rate between the segments at 1 O3O' and 11 2O' on the
northern EPR must not affect the underlying plumbing system.
Acknowledgements
We wish to thank Charles Shearer and Stanley Mertzman for their help with the
SIMS and XRF work, respectively. We thank Abigail Walker and Claire Lunch for their
analytical support. This work was supported by NSF grant OCE-9730079.
94
LINKING THE LOCAL, REGIONAL AND GLOBAL PETROLOGIC
SYSTEMATICS OF MID-OCEAN RIDGES: THE SOUTHEAST INDIAN RIDGE
CASE
Rachel Sours-Page
David M. Christie
David W. Graham
Roger L. Nielsen
Laura Magde
MS in preparation for submission to Geochimica et Cosmochimica Acta
July, 2000, 44 pages
95
Abstract
The SEIR exhibits greater than 50% of the global range in axial morphology,
geophysics, and Na8, Fe8 and CaO/Al203 at nearly invariant spreading rate. Given that
the current paradigm of mid-ocean ridge basalt formation implies that such variations are
representative of different spreading rate, the Southeast Indian Ridge provides a natural
laboratory to study the controls on crustal accretion at constant spreading rate.
Major element analyses were performed on 78 dredges and 22 wax cores and used
to distinguish 181 distinct chemical groups along the Southeast Indian Ridge between 88°
and 11 8°E. Chemical groups were found to represent 155 N-MORB, 26 E-MORB and 2
FeTi basalts within 154 axial and 27 seamount groups. Mineralogy within the samples
varies from plagioclase to plagioclase + olivine ± clinopyroxene.
N-MORB glass compositions range from 6.2 to 9.8 wt % MgO, and vary in
calculated degree of crystallization from 9-68%. Crystallization models suggest that 2-3
kb equilibrium crystallization most accurately reproduces the data. K20, P205, Na20,
Si02 and K20/Ti02 all increase, while CaO, FeO, and CaO/Al203 decrease with
increasing axial depth. MgO, Al203, Ti02 and extent of crystallization do not vary
systematically with depth.
Between 88° and 11 8°E, Na8, Feg, and CaO/Al203 exhibit slopes that are
shallower than and statistically distinct from, the global aiTay published in Klein and
Langmuir (1987) when plotted versus depth. (The equations for these parameters were
not published in K+L but were later estimated by Brodholt and Batiza (1989). The
Southeast Indian Ridge data are generally higher in Nag and CaOIAl2O3, and lower in Fe8
than the array. Segments progess non-systematically from high Na8 and low Fe8 in the
east to low Na8 and high Fe8 in the west as axial depth increases. These correlations are
suggestive of higher pressures and extents of melting in the west and consistent with the
current belief that mantle temperatures are higher in the western part of the study area
and grade to lower temperatures in the east.
Combined, ridge segments along the Southeast Indian Ridge, Amsterdam-St. Paul
Plateau and Australian-Antarctic Discordance exhibit coherent, positive correlations
96
between Na8, longitude and depth. The combined dataset overlaps the Global Array, yet
is offset to higher values of Na8 and has a steeper slope. The Amsterdam-St. Paul and
SEIR data have individual trends that are oblique to both the combined data and the
Global Array, yet are sub-parallel to one another.
Based on our observations of the Southeast Indian Ridge, it is apparent that the
SEIR results are not simply predicted from the global Klein and Langmuir model. The
SEIR deviates significantly from the global array. Possible reasons for this include that
the Global Array is heavily weighted by samples from the Mid-Atlantic Ridge and East
Pacific Rise. Because the Indian Ocean is significantly younger than the other ocean
basins, and is isotopically distinct, it may also be chemically distinct, either from
incomplete mantle mixing or from more frequently encountered chemical heterogeneities.
Introduction
The global model of mid-ocean ridge petrology developed over the past 30 years
is based on the assumption that observed differences in accretionaiy processes (those
responsible for crustal thickness, structure and chemistry) are a function of seafloor
spreading rate and mantle temperature. Extensive research performed on the East Pacific
Rise(EPR) and the Mid-Atlantic Ridge(MAR) has shown that there are fundamental
differences between fast and slow-spreading mid-ocean ridge environments which
support this contention (Macdonald 1982; Klein and Langmuir 1987; Sinton and Detrick
1992; Malinvemo 1993). The current paradigm holds that fast spreading ridges are
driven by wanner mantle temperatures, allowing for shallower axial depths, higher
extents of melting, and thicker crust; whereas slow spreading ridges are characterized by
cooler mantle temperatures, lower extents of melting, and thinner crust. Due to these
extreme differences, the EPR and MAR are usually taken as endmembers and the
remainder of the global system is placed within their context. Recent evidence from
intermediate spreading ridges, however, raises questions about our assumptions, and
97
indicates that this paradigm of simple linear spreading rate dependence needs to be reevaluated.
The Southeast Indian Ridge (SEIR) between 88 and 11 8E is a unique area in
which to study the parameters controlling crustal accretion. Along its 2500 km length, it
encompasses nearly the entire known variability in inferred upper mantle temperature and
axial morphology, yet is largely uninterrupted by major transforms and intervening
hotspots. No other comparable length of the global spreading system is characterized by
such a range. All of these variations occur at nearly constant spreading rate along the
SEIR (from 69 to 75 mm!yr, full rate).
In the Austral summer of 1995, integrated bathymetric, geophysical and
petrological/geochemical surveys (WWO9 and 10) characterized 2500 km of previously
unsampled ridge axis on the SEIR between 88 and 120°E. The major objectives were (1)
to investigate changes in axial morphology and axial depth; (2) to determine where melt
supply and distribution occurs, both within and between segments, using gravity data;
and (3) to investigate the geochemical variability along axis and its relation to changes in
axial ridge morphology. The morphological and geophysical data have been presented
by Ma and Cochran (1996), Cochran et al. (1997), Sempere et al. (1997), West et al.
(1997), West and Sempere (1998), Small et al. (1998). This paper focuses on the new
geochemical results from the SEIR, and, in particular:
How variations in chemical composition (including the nature and extent of
compositional diversity) of axial lavas are related to axial morphology and other
geophysical characteristics.
How the petrologic features observed on the SEIR compare to predictions based on
the current global paradigm (e.g. Klein and Langmuir (1987)).
Geolo!ic Setting
The Southeast Indian Ridge extends from the Rodrigues/Indian Ocean Triple Junction to
the Macquarie Triple Junction in the Indian Ocean south and west of Australia (Fig 21).
98
It separates the Indo-Australian and Antarctic Plates and has full spreading rates between
57.5 and 80 mm/yr (DeMetset al. 1990). The study area between 88° and 1 18°E is
divided into 16 segments (C2-C17), as determined by Cochran et al. (1997) on the basis
of topography and geophysics, yet it is mostly free of major fracture zones offsets,
hotspot influenced areas, or other perturbations (Fig 21). This unique section of the
global ridge system spans from 800 km east of the hot mantle near the Amsterdam-St.
Paul Hotspot (ASP) in the west, to the relatively cold mantle underlying the Australian-
Antarctic Discordance (AAD) in the east (Forsyth et al. 1987; Marks et al. 1991). The
differences in temperature between these two regions is presumed to be the main driving
force for the range of ridge characteristics represented on the Southeast Indian Ridge
(Klein et al. 1991; Sempere etal. 1997).
Due to its rise-type axial morphology and relative proximity to the hotspot, the
western boundary of the study area is thought to be underlain by relatively hot mantle.
The influence of hotspot mantle underlying Amsterdam and St. Paul Islands is generally
restricted to within ± 200km of the ASP Plateau, as evidenced by the elevated ridge
depths, ridge morphology, and isotopic signatures such as high 3He/4He (Graham et al.
1999). To the west, near 88°E, the ridge gets shallower and shows a broad (long
wavelength) minimum of appoximately 2300 m, and exhibits morphology most similar to
the East Pacific Rise.
The AAD is situated in the eastern part of the SEIR, 400 km to the east of the
study area, and was first noted for its anomalously deep axial depths (- 5000 m). Despite
its intermediate spreading rate, this region is inferred to have low mantle temperatures,
low extents of melting , and low rates of melt production, consistent with its wide rift
valley and the segmentation characteristics of a slow-spreading center (Anderson et al.
1980; Klein et al. 1991; Christie et al. 1990; Sempere etal., 1991; Palmer etal., 1992).
Although spreading rate is nearly invariant, other physical parameters change
dramatically and non-uniformly along the SEIR axis. Generally, the study area changes
in character from more similar to the fast-spreading East Pacific Rise near 88E, to more
like the slow-spreading Mid-Atlantic Ridge near the AAD. Most notably, axial depths
vary from only 2300 m on Segment C17 to more than 4400 m on Segment C2, a
110
I
N
oQE Longitude
130°
100
difference of 2100 m over 2500 km (Fig 21). Ridge segmentation changes from "super
segments," i.e. C 14-17, in the west to smaller segments in the east (Small et al. 1999).
Axial morphology also varies from EPR-like axial highs in the west (82°-i 04°E) through
a transition zone characterized by shallow valleys (104°-il 4°E) to deep MAR-like
valleys from 114° to 128°E (Ma and Cochran 1996). Crustal thicknesses are estimated to
decrease from 7-8 km at 88°E to 4 km at 1 18°E (Sempere et al. 1997). The gravity field
shifts from a rough, high amplitude signal in the east to a smooth field in the west (Small
and Sandwell 1989, 1992). Within this region, two "bull's eye" type mantle Bouguer
anomalies are centered at 100.5 °E and 107.5 °E, indicating that either the crust is thicker
or the mantle is hotter, or both, in these areas (Fig 21). These bull's eyes are similar in
character to those found on the Mid-Atlantic Ridge, but are of smaller magnitude (West
and Sempere 1998). Notably, along the SEIR, they are spatially related to the more EPRlike topographic highs.
How large is the mantle temperature gradient?
Topography, lithospheric rheology, gravity and crustal thickness variations are all
indicative of a relatively large mantle temperature gradient beneath the Southeast Indian
Ridge. Estimates for the potential magnitude of the temperature change range from 60°C
by Sempere et al. (1997), calculated to be consistent decrease in crustal thickness, to
150°C reported by Yan et al. (1989) at a depth of 150 km between the Amsterdam-St.
Paul Hotspot and the AAD using lateral velocity heterogeneities in the upper mantle.
Furthermore, using the Klein and Langmuir model of axial depths and crustal thicknesses
based on isostatic compensation, Sempere et al. (1997) calculated a temperature
difference of 60°-80°C over the same area, assuming a depth of compensation of 200 km
and a constant final pressure of melting. Regardless of the exact number, such variations
in mantle temperature have important implications on melt production and melting
regime processes.
101
Petrologic expectations
Klein and Langmuir (1987), hereafter refered to as K+L, recognized that on a
global scale, there is a systematic relationship between the major element chemistry of
MORB and the regional depth of the axis from which they erupt. The best known
expression of this relationship is the global correlations of Na8, Fe8 and CaO/Al203
values of lavas with axial depth. The variations in Na8, Fe3 and CaO/Al203 are attributed
to regional changes in source composition, extents, and pressures of melting related to
mantle temperature and/or heterogeneities where higher mantle temperatures result in
more extensive melting (low Nag) and higher mean pressures of melting (high Feg).
The large gradient in mantle temperature that one would presume from the physical
characteristics along axis leads one to expect that the major element compositions of
MORB along the SEIR would be correlated with changes in regional depth.
Incompatible elements such as Al, Na, Ti, P, and K should increase from west to east,
while the compatible elements, such as Ca, Mg, and Fe should decrease over the same
interval, indicating that the extent and pressure of melting decrease from west to east.
Methods
Major element analyses were performed using the CAMECA SX-50 Electron
Microprobe at Oregon State University (Table 6). Three point analyses from each glass
chip were averaged. Back-scatter electron images were used to avoid the effects of any
microcrystalline textures in the glass chips. Analyses were performed using a 30 nA
beam current, a 15 kV accelerating voltage, and a 3-5 t defocused beam. Calibrations
were performed using Smithsonian standards USNM 113498/1 (Makaopuhi Lava) for Si,
Al, Fe, Ca, and Ti, USNM 133868 (Kakanui Anorthoclase) for Na, USNM 143966
(Microcline) for K, and USNM 122142 (Kakanui Augite) for Mg (Jarosewich et al.,
1980). Na was counted first due to its susceptibility to beam damage (Nielsen et al.,
1995a). Na, Ca, and Mn were counted for 10 seconds and Mg, Al, Si, Ti, and Fe were
Table 6. Chemical group averaged major element analyses for Southeast Indian Ridge lavas
P67.3
P68
P70
D71.1
90.80
42.89
P71.2
P72.1
90.80
42.89
91.29
43.42
2350
n = 14
2350
1605
91.09
43.08
91.09
43.08
89.32
41.72
89.32
41.72
89.32
41.72
89.19
41.89
89.19
41.89
89.19
41.89
89.19
41.89
88.53
41.82
D69
88.92
41.87
2493.5
2493.5
1570
1570
1570
1942.5
1942.5
1942.5
1942.5
3779
2353
n=21
n=21
n=5
0=4 n=41
N
n=2 n=2 n=4 n=4
N N, SM N,SM N,SM N,SM N, SM N,SM N,SM
50.90
49.79
50.29
50.29
50.33
50.37
50.05
50.78
50.71
51.00
50.64
1.72
1.89
1.48
1.49
1.47
1.10
1.17
1.15
0.99
2.30
N
48.86
0.92
N
50.61
FeTi
50.82
N
50.40
1.71
1.39
1.62
1.20
Al203
14.82
13.89
15.12
15.59
15.19
15.97
15.22
15.15
15.42
13.99
17.61
14.91
15.62
14.89
15.17
FeO
MnO
MgO
CaO
Na20
10.15
11.26
10.17
10.08
10.27
8.52
8.94
8.81
8.78
12.34
8.28
10.36
9.02
9.77
9.20
0.13
0.15
0.14
0.13
0.14
7.40
0.13
0.14
0.12
0.18
0.12
0.14
0.13
8.40
8.72
6.95
9.83
7.69
0.12
8.07
0.13
7.95
0.14
8.28
7.56
8.30
12.55
12.89
12.93
12.60
13.13
12.47
11.84
11.93
11.83
12.57
2.89
2.48
0.08
2.48
2.44
2.19
10.62
2.91
2.67
2.87
2.88
2.49
0.08
0.08
0.07
0.17
2.24
0.05
0.13
0.13
0.06
0.08
0.08
0.06
0.18
0.13
0.11
0.07
0.06
0.06
0.03
0.06
0.04
0.05
0.05
0.16
0.14
0.05
99.94 100.48 100.50 100.41 100.01
100.00
99.88
Dredge
Longitude (°E)
Latitude (°S)
Depth
# Analyses
Type
Si02
Ti02
D65.1
D65.2
D66.1
n = 44
P66.2
n=1
7.67
6.80
7.68
6.94
11.51
11.24
12.30
2.66
2.87
2.99
12.64
3.11
P66.3
P67.1
D67.2
D67.4
n=1
90.20
42.57
2575
n=8 n = 20
N N,SM
0.11
0.11
0.16
0.10
0.09
0.09
0.08
0.05
0,01
0.05
0.05
0.04
0.08
0.06
99.59
99.33
99.91
100.51
100.43
99.59
99.75
98.97
Na8
2.54
2.42
2.88
2.71
2.67
2.47
2.59
2.60
2.46
2.53
2.93
2.56
2.89
2.72
2.61
Fe8
9.60
9.26
9.64
8.31
9.27
8.44
9.40
9.48
9.99
10.59
11.33
9.85
9.12
9.69
CaO/Al203
0.78
0.81
0.81
0.81
0.83
0.81
0.85
0.83
0.85
0.76
0.71
0.79
K20/Ti02
0.08
0.08
0.07
0.07
0.07
0.07
0.07
0.07
0.07
0.07
0.06
0.08
0.76
0.09
9.04
0.79
0.10
K20
0.14
0.16
P205
0.13
Cr203
Total
0.08
0.05
0.83
0.05
Table 6 (cont'd)
Dredge
Longitude (°E)
Latitude (°S)
Depth
# Analyses
Type
Si02
D77
078
93.11
93.11 93.77
43.88
43.88
43.88 44.12
94.83
44.83
2585
2652.5
2652.5
n=5 n= 13
n=2
n=2
n= 1
N
51.57
N
51.23
N
51.48
N
51.21
N
50.74
1.67
1.73
1.62
1.71
1.67
1.07
2.30
1.89
1.35
073.1
073.2
074.1
074.2
075.1
075.2
076.1
076.2
91.29
43.42
D72.5
91.29
43.42
91.68
43.47
9 1.68
92.33
43.43
92.68
43.58
92.68
43.58
93.11
43.47
92.33
43.43
1605
1605
2737.5
2737.5
2560
2560
2585
n=2 n=1 n= I n= 1
N,SM N,SM N, SM N,SM
n=8
n=4
n=4
N
50.50
N
51.23
1.85
D72.2
91.29
43.42
072.3
072.4
91.29
43.42
1605
1605
076.3
2652.5
2795
2719
n= I n=6 n=10
N
N
N
50.40
51.06
51.11
51.70
51.17
T102
1.31
1.21
1.25
1.16
FeTi
50.83
2.39
Al203
14.97
15.31
15.11
15.76
13.60
14.34
14.52
14.83
15.09
14.83
15.23
16.33
14.71
14.62
15.47
FeO
MnO
9.61
9.45
9.53
9.17
12.02
10.59
9.92
9.70
9.58
9.81
9.50
11.91
10.15
8.68
0.13
0.19
6.87
0.12
0.12
7.28
8.08
10.67
2.34
11.11
11.89
2.89
2.77
0.14
MgO
CaO
Na20
K20
52.39 50.64
3.33
3.09
8.15
0.16
8.69
12.66
2.64
0.20
0.21
0.07
0.19
0.14
0.16
0.18
0.08
0.20
0.11
0.04
0.16
0.04
101.81 99.03
99.06
0.13
0.13
0.12
0.12
0.17
0.16
0.16
0.14
0.15
0.16
7.87
8.60
8.11
8.11
6.26
7.17
7.24
7.20
7.29
7.43
12.58
12.46
12.97
10.61
11.81
11.93
11.79
11.59
11.69
2.63
2.60
12.74
2.63
2.55
2.99
2.91
2.91
3.14
7.48
11.70
3.28
0.21
0.08
0.06
0.06
0.05
0.20
0.14
0.15
0.09
0,07
0.07
0.07
0.24
0.16
0.15
0.18
0.16
0.15
0.05
0.06
0.04
0.04
0.02
0.05
0.04
0.05
0.05
0.03
0.05
0.06
100.38
101.07
101.36
101.17
99.35
99.68
99.91
100.54
100.48
100.59
100.38
100.66
Na8
2.59
2.83
2.67
2.60
2.35
2.61
2.63
2.84
3.09
3.07
2.88
2.90
1.92
2.62
2.80
Fe8
9,39
10.46
9.71
9.35
9.13
9.20
8.65
8.38
8.72
8.62
8.55
9.30
10.03
8.94
8.81
CaO/Al203
0.84
0.81
0.84
0.82
0.78
0.82
0.82
0.80
0.78
0.78
0.77
0.78
0.73
0.76
0.77
K20/Ti02
0.06
0.05
0.05
0.05
0.08
0.08
0.09
0.12
0.10
0.12
0.12
0.06
0.08
0.07
0.10
Cr203
Total
0.05
Table 6 (cont'd)
Dredge
Longitude (°E)
Latitude (°S)
Depth
# Analyses
Type
Si02
Ti02
D88.1
D88.2
D89.1
96.12
45.20
D87
96.37
46.78
96.83
47.08
96.83
47.08
97.51
47.45
2607.5
2520
2567.5
2567.5
2470
n=1 n=25 n=38 n=17
n=5
n=1
N
50.64
N
50.84
N
51.05
E
51.20
D81
D82.1
D82.2
D83.1
D83.2
D84.1
D84.2
D85
95.50 95.59
45.14 45.17
95.70
45.18
95.70
45.18
95.76
45.05
95.76
45.05
95.93
45.11
95.93
45.11
3190
3163.5
3163.5
3069
3069
2660
2660
n=5 n=13 n=11 n=3 n=110
n=1
n = 24
N
E
51.63
50.87
N
51.15
50.91
D79.1
D79.2
95.41
45.11
95.41
45.11
3045
3045
N
50.97
N
50.47
D80
3142.5
n=2 n=29
N
N
51.04 50.46
N
50.91
50.20
E
50.60
1.54
1.49
1.42
1.33
1.31
1.35
1.37
1.45
1.38
1.17
1.70
1.75
14.72 14.68
15.45
15.27
16.29
15.56
15.64
15.32
15.46
15.32
15.16
15.24
16.94
N
1.59
N
1.66
1,52
Al203
15.05
15.24
FeO
MnO
MgO
CaO
Na20
K20
P205
Cr203
9.62
0.13
9.25
9.77
9.48
9.22
8.63
7.98
8.45
8.45
8.84
8.93
9.34
8.98
10.10
8.69
0.12
0.14
0.15
0.13
0.10
0.11
0.10
0.12
0.13
0.12
0.14
0.13
0.15
0.13
8.26
7.03
6.66
7.69
7.94
7.57
7.59
8.30
8.31
8.00
11.79
11.91
12.17
12.22
11.85
11.37
11.72
2.82
2.76
2.87
2.78
2.63
2.67
3.16
7.94
12.09
3.16
8.07
8.24
7.98
7.93
11.78
2.94
11.81
11.81
11.95
12.75
11.66
11.41
2.95
2.79
2.81
2.43
2.78
0.16
3.25
0.07
0.08
0.13
0.06
0.04
0.22
0.03
0.51
0.12
0.15
0.11
0.12
0.12
0.13
0.12
0.38
0.21
0.13
0.15
0.18
0.12
0.11
0.12
0.12
0.14
0.15
0.12
0.11
0.11
0.12
0.06
0.06
0.05
0.06
0.06
0.05
0.05
0.06
0.05
0.05
0.04
0.10
0.10
0.05
100.04
99.49
100.15 99.19
100.28
98.28
99.75
100.62
99.51
100.12
99.78
99.76
99.95
100,03
100.78
Na8
2.71
2.74
2.71
2.63
2.75
2.79
3.16
3.14
2.97
3.04
2.78
2.79
2.54
2.42
2.75
Fe8
9.15
9.05
8.79
9.72
9.14
7.98
8.35
8.56
9.23
8.89
9.22
9.42
8.49
6.46
CaO/Al203
9.10
0.78
0.78
0.83
0.83
0.77
0.74
0.72
0.78
0.75
0.77
0.76
0.78
0.84
0.77
0.67
K20/Ti02
0.07
0.07
0.08
0.08
0.08
0.08
0.29
0.16
0.10
0.11
0.12
0.07
0.06
0.09
0.29
Total
Table 6 (cont'd)
Dredge
Longitude (°E)
Latitude (°S)
Depth
# Analyses
Type
Si02
Ti02
D89.2
D89.3
D90.1
D90.2
D90.3
D90.4
D91
97.51
97.51
98.16
47.45
47.71
98.16
47.71
98.16
47.71
98.60
47.45
98.16
47.71
47.91
D92
98.94
48.10
2470
2470
2607
2607
2607
2607
2900
2667.5
n=43 n = 38
n=2
n=2 n= 11
D93.1
99.12
47.98
D93.2
99.12
47.98
D93.3
99.12
47.98
2380
2380
2380
D93.4
D94
D95
99.12
47.98
99.25
47.75
99.41
D96
100.67
47.55 47.34
2380 2390.5 2222.5
2465
n=1
n=2 n= 11 n=2 n=8 n = 13
n=1 n=15 n = 21
N N, SM N,SM N,SM N,SM N,SM N, SM
N
N
n=9
49.73
N
52.34
50.39
N
49.67
48.80
50.92
1.90
1.49
1.45
2.50
1.32
1.04
1.10
0.84
0.90
1.04
1.11
1.82
15.36
16.02
17.06
16.68
16.81
16.47
14.68
E
50.60
E
N
N
51.43
49.91
1.66
1.55
1.64
50.88
51.04
49.67
49.95
50.17
51.24
Al203
16.25
15.30
15.18
14.85
16.19
16.35
14.09
15.56
FeO
MnO
8.85
8.79
10.62
9.91
8.67
8.67
11.75
8.85
8.40
9.11
7.78
8.18
8.49
8.39
10.35
0.12
0.12
0.14
0.16
0.11
0.14
0.16
0.14
0.11
0.14
0.10
0.10
0.10
0.11
0.12
7.95
7.49
7.20
7.31
8.22
7.86
6.66
8.32
8.70
8.61
9.75
9.39
8.99
8.81
6.98
11.07
12.00
11.12
11.49
11.88
10.30
12.17
12.18
12.96
12.77
12.93
12.65
12.43
11.30
2.85
3.06
2.65
2.41
2.47
2.15
2.18
239
2.60
3.25
0.06
0.08
0.05
0.06
0.08
0.26
0.06
0.08
0.09
0.09
0.06
0.06
0.01
MgO
CaO
Na20
K20
3.10
3.11
3.21
3.15
12.06
2.96
0.49
0.19
0.32
0.16
0.13
0.14
0.15
0.14
0.28
0.14
0.12
0.05
0.05
0.22
0.04
0.11
0.05
0.14
0.05
0.14
0.04
0.16
0.05
0.07
0.09
0.06
0.05
0.05
0.06
0.08
0.06
100.31
100.32
99.32
99.51
99.69
98.34
99.98
100.20
98.15
101.63
100.28
100.50
100.86
Na8
3.08
2.93
2.92
2.89
3.04
2.80
2.77
2.77
2.90
2.87
7.95
9.28
8.75
9.03
8.43
2.70
10.12
2.70
8.76
2.68
9.57
2.80
Fe8
2.56
9.53
10.69
10.49
10.14
9.73
8.65
0.75
0.07
0.75
0.77
0.08
0.14
Cr203
Total
9.38
CaO/Al203
0.68
0.78
0.73
0.77
0.74
0.73
0.73
0.78
0.79
0.81
0.75
K20/Ti02
0.30
0,21
0.08
0.07
0.10
0.10
0.11
0.11
0.07
0.06
0.06
0.78
0.07
0.17
101.39 101.28
Table 6 (cont'd)
Dredge
Longitude(°E)
Latitude (°S)
Depth
#Analyses
D97
098.1
098.2
D98.3
099
0100.1
0100.2
D100.3
0100.4
0101.1
100.86
47.19
100.96
47.46
100.96
100.96 101.22
47.59
47.63
101.53
47.63
101.53
47.46
101.53
47.63
101.53
47.46
47.63
2015
2583
2583
2583
2842
2856.5
2856.5
2856.5
n=10 n11
n=3
n=2 n=15
n=4
n=10
N
51.18
N
51.79
E
51.15
0101.3
D101.4
101.86
47.77
D101.2
101.86
47.77
101.86
47,77
101.86
47.77
2856.5
2780
2780
2780
2780
n=9
n=4
n=2
n=4
n=11
n=1
N
51.24
N
51.52
N
51.72
N
51.93
N
51.51
N
51.29
N
50.84
Type
Si02
Ti02
N
49.12
51.43
E
50.38
1.27
1.39
1.42
1.48
1.55
1.66
2.02
1.86
2.46
1.58
1.93
1.51
1.40
Al203
17.92
15.05
15.63
14.80
14.68
15.01
14.50
14.77
13.98
14.74
14.34
15.42
15.86
FeO
MnO
MgO
CaO
Na20
K20
8.04
9.05
8.70
9.41
9.85
9.33
10.34
10.03
11.38
9.49
10.27
9.06
8.61
0.13
0.14
0.13
0.16
7.70
0.17
0.15
0.16
0.14
0.16
0.13
0.15
0.13
7.33
7.56
6.93
7.30
6.20
7.49
7.04
7.84
0.16
8.47
11.94
2.85
11.80
2.97
11.82
11.37
1140
10.30
11.83
11.17
12.06
2.99
3.14
3.07
3.31
2.98
3.05
2.83
11.85
2.83
0.22
0.18
0.14
0.14
0.05
0.06
N
8.74
7.85
7.87
11.48
12.15
12.04
3.24
2.77
2.88
0.12
0.14
0.24
0.16
0.15
0.29
0.25
0.19
0.29
0.15
0.10
0.13
0.14
0.13
0.19
0.06
0.02
0.05
0.17
0.04
0.15
0.04
0.22
0.03
0.26
0.04
0.14
0.03
0.01
0.04
0.22
0.18
0.02
100.19
100.14
99.50
99.85 100.45
100.20
100.22
100.49
100.08
100.51
99.87
100.55
100.40
Na8
3.52
2.72
2.83
2.74
2.73
2.83
2.75
2.81
2.64
2.79
2.70
3.01
Fe8
9.27
8.80
8.48
8.91
8.74
8.60
8.57
8.86
8.39
8.64
8.66
2.78
8.80
CaO/Al203
0.64
0.81
0.77
0.81
0.79
0.78
0.77
0.74
0.80
0.78
0.78
0.75
K20/Ti02
0.10
0.10
0.17
0.11
0.80
0.10
0.17
0.13
0.10
0.12
0.09
0.11
0.14
0.13
Cr203
Total
9.39
Table 6 (cont'd)
Dredge
Longitude (°E)
Latitude (°S)
Depth
# Analyses
Type
Si02
D102
D103.1
D103.2
D103.3
101.86 102.14
47.77 47.88
102.54
102.54
48.02
102.54
48.02
2986
3230
3230
n=4 n = 24
n=3
n=1
N
51.45
N
51.27
N
51.64
D1O1.5
2780
N
51.98
D105
D106.1
D106.2
D106.3
D106.4
D106.5
D107
103.08 103.04
103.35
47.85
D104
103.25
103.25
103.25
103.25
103.25
48.02
47.92
47.77
47.90
47.90
47.90
47.90
47.90
3230
2401
2783
2985
2985
2985
2985
n=1 n=5 n = 13
N
N N,SM
n=1
n=1
n=9
n = 13
n=2
n=7
N
50.76
N
50.90
N
51.67
N
51.25
N
51.09
2985 2787.5
51.53
50.66
51.07
N
50.70
1.54
1.11
1.12
1.11
1.18
1.17
1.40
hO2
2.14
1,33
1.52
1.42
1.17
1.07
Al203
14.22
14.87
15.50
15.03
15.98
16.17
15.37
15.60
15.74
15.67
15.29
15.21
14.74
FeO
MnO
10.85
9.19
9.27
9.25
8.44
8.33
.8.49
8.44
8.19
8.72
8.71
9.27
0.14
0.14
0.13
0.12
0.13
0.13
9.09
0.14
0.15
0.15
0.11
0.14
0.14
7.45
MgO
CaO
Na20
K20
Cr203
Total
Na8
6.60
7.91
7.92
7.90
8.60
8.63
7.33
8.35
7.98
0.08
8.55
8.25
8.04
12.25
12.35
12.29
12.59
12.38
11.85
2.46
2.51
2.60
2.66
2.57
10.69
12.57
12.05
12.46
12.54
12.62
3.26
2.68
2.88
2.71
2.56
2.71
11.50
3.37
0.23
0.11
0.11
0,12
0.09
0.06
0.17
0.12
0.14
0.12
0.08
0.08
2.86
0.13
0.19
0.11
0.13
0.12
0.08
0.08
0.08
0.18
0.10
0.09
0.08
0.11
0.05
0.16
0.10
0.08
0.06
0.16
0.06
0.06
0.07
0.04
101.16 100.51
99.84
99.49
99.44
99.68
100.71
99.70
99.09
2.95
3.13
2.59
2.51
2.81
2.76
9.13
2.58
2.66
9.11
8.77
8.35
0.80
0.09
0.06
0.06
0.06
100.32 100.42
100.84
100.84
2.85
2.68
0.02
2.74
2.65
2.79
Fe8
8.52
9.04
9.13
9.09
9.44
9.37
7.97
9.08
8.41
CaO/Al203
0.75
0.85
0.78
0.83
0.79
0.78
0.75
0.79
0.78
0.78
0.82
0.81
K20/Ti02
0.11
0.08
0.07
0.09
0.08
0.06
0.11
0.10
0.12
0.11
0.07
0.07
Table 6 (cont'd)
Dredge
Longitude (°E)
Latitude(°S)
D108.1
D108.2
DuO.!
D11O.2
Dill.!
D1!1.2
Dl1l.3
D112
D113.l
103.57
103.57
47.97
103.93
103.93
104.66
104.66
104.66
104.97
48.10
48,10
48.21
48.21
48.21
48.32
105.22
48.75
47,97
D113.2
105.22
48.75
D114.l
D114.2
105.59
105.59
49.12
49.12
D114.3
105.59
49.12
3080
3080
3480
3480
3069.5
3069.5
3069.5
3162.5
3630
3630
2875
2875
2875
n=8
n=1
n=22
n=3
n=15
n=1
n=3
n=5
n=5
n=10
n=21
n=1
n=1
N
50.93
N
50.93
N
E
51.36
E
51.54
E
51.56
N
E,SM
51.17
N
51.16
N
51.54
N
51.58
51.06
49.78
E, SM
49.96
E, SM
50.12
1.12
1.12
1,28
1.97
1.73
1.69
1.64
1.34
1.53
1.36
1.62
1.78
1.54
15.72
15.71
14.94
14.41
15.82
15.84
15.98
15.42
15.11
15.46
16.97
16.32
17.23
F'eO
8.37
8.35
9.05
10.62
8.61
8.64
8.63
8.72
9.44
8.95
8.26
7.99
MnO
MgO
CaO
Na20
K20
0.12
0.12
0.12
0.12
0.11
7.47
7.92
7.53
7.90
7.68
12.31
12.41
12.36
10.76
11.28
11.36
11.30
12.19
11.89
12.05
11.22
2.51
2.51
3.07
3.44
3.42
3.38
2.85
2.82
2.78
3.20
0.12
0.12
2.60
0.09
0.08
7.08
11.14
3.21
0.08
7.08
0.12
7.25
0.13
8.49
0.16
6.87
0.12
8.81
0.12
7.92
7.84
0.09
0.17
0.43
0.37
0.35
0.15
0.11
0.74
0.82
0.69
P205
0.10
0.10
0.18
0.19
0.11
0.26
0.25
0.06
0.04
0.05
0.06
0.05
0.10
0.06
0.25
0.03
0.17
0.04
0.18
0.06
0.09
0.05
0.13
0.13
0.04
0.07
0.04
100.17
99.92
100.04
99.82
100.09
100.43
100.66
100.04
99.91
99.93
99.42
98.95
100.80
Na8
2.82
2.70
2.57
2.65
3.10
3.14
3.18
2.83
2.65
2.75
3.08
2.87
3.43
Fe8
9.72
9.17
8.93
8.74
7.07
7.40
7.73
8.58
8.66
8.78
7.30
6.72
8.68
0.83
0.75
0.71
0.72
0.71
0.79
0.79
0.78
0.66
0.68
0.65
0.07
0.08
0.25
0.22
0.21
0.11
0.08
0.08
0.46
0.46
0.45
Depth
#Analyses
Type
Si02
Ti02
Al203
Cr203
Total
CaO/Al203
0.78
K20/Ti02
0.10
0.79
0.11
8.42
11.17
3.27
Table 6 (cont'd)
Dredge
Longitude (°E)
Latitude (°S)
Depth
#Analyses
Type
Si02
D114.4
D115.1
D115.2
D115.3
D116.1
D116.2
D116.3
D117.1
D117.2
Dl 18
D119.1
107.15
48.35
107.53
48.43
107.50
48.35
2349.5
D119.2 D120
107.50 107.81
48.35 48.55
105.59
49.12
105.87
105.87
49.23
106.49
106.49
106.49
49.23
105.87
49.23
107.15
48.87
48.87
48.87
48.35
2875
3635.5
3635.5
3635.5
4835
4835
4835
3520
3520 2672.5
n= I
n=2
n=5
n=1
n=9
n=1
n=8
n=7
n=2
n=7
n=1
E,SM
N
51.37
E
E
51.02
E
50.42
50.79
E
51.74
E
50.65
E
50.85
E
50.07
N
51.89
51.39
E, SM
50.83
E, SM
50.91
51.51
1.54
1.47
1.59
1.59
1.62
1.63
1.47
1.90
2.58
2.07
1.90
1.83
1.37
Al203
17.31
15.34
14,92
15.65
15.72
15.82
16.22
15.24
14.28
14.99
15.56
15.60
15.50
FeO
MnO
MgO
CaO
Na20
K20
P205
Cr203
7.85
9.21
9.44
8.63
8.74
8.88
8.20
9.69
11.08
9.92
9.71
9.40
8.79
0.10
0.13
0.13
0.13
0.11
0.11
0.14
0.16
0.14
0.13
8.11
0.10
7.75
7.36
8.35
8.17
8.50
8.76
6.77
5.30
6.35
6.19
0.14
6.64
0.10
7.64
11.19
11.67
11.62
11.31
11.25
11.36
11.42
10.69
10.60
10.91
11.18
11.86
3.27
2.87
3.17
3.23
3.18
3.17
3.07
3.53
9.43
3.77
3.58
3.70
3.54
3.16
0.69
0.19
0.18
0.30
0.29
0.29
0.47
0.43
0.58
0.55
0.17
0,22
0.15
0.17
0.17
0.30
0.17
0.33
0.14
0.18
0.19
0.29
0.22
0.21
0.21
0.05
0.06
0.05
0.05
0.05
0.05
0.05
0.03
0.02
0.03
0.02
0.03
0.12
0.05
100.39
100.68
99.97
100.06
100.16
101.02
100.24
99.25
99.10
99.72
99.75
100.02
100.27
Na8
3.31
2.78
2.93
3.36
3.25
3.36
3.36
3.07
2.97
3.03
3.04
3.03
Fe8
8.04
8.79
8.38
9.22
9.02
9.47
7.64
7.17
6.70
7.15
8.20
CaO/Al203
0.65
0.76
0.78
0.72
0.72
9.70
0.72
2.76
6.59
0.70
0.70
0.66
0.71
0.70
0.77
K20/Ti02
0.45
0.13
0.11
0.19
0.18
0.18
0.23
0.15
0.18
0.21
0.31
0.72
0.30
T102
Total
2349.5 2803.5
n= 16 n = 20
N
0.12
Table 6 (cont'd)
Dredge
Longitude (°E)
Latitude (°S)
Depth
# Analyses
Type
Si02
Ti02
D122.1
108.28
48.74
D124
D125.1
D125.2
D126
D127
D128.1
D128.2
D128.3
D128.4
D128.5
108.28 108.47 108.51
48.74 48.80 49.03
109.11
109.11
109.48
109.88
110.41
110.41
110.41
110.41
110.41
49.45
49.45
49.53
49.66
49.83
49.83
49.83
49.83
49.83
D122.2
D123
3277
3277
3392
3935
3475
3475
3240
3257.5
3745
3745
3745
3745
3745
n = 15
n=1
n = 13
n = 14
n=7
n=1
n=7
n=9
n=3
n=11
n=4
n=3
n=2
N
N
51.62
N
51.37
N
52.10
N
51.95
E
50.80
E
51.24
E
51.75
N
51.68
50.14
E
50.22
N
50.94
N
50.88
N
50.88
2.00
2.04
1.58
1.18
1.28
1.32
1.49
1.48
1.20
1.18
0.92
0.91
Al203
14.88
14.77
15.28
15.74
15.66
15.39
17.67
16.86
17.36
17.45
17.13
17.25
0.90
17.30
FeO
MnO
MgO
10.14
10.13
9.43
8.75
8.62
8.65
8.05
8.19
8.52
8.61
7.46
7.61
7.62
0.14
0.14
0.12
7.29
0.13
0.12
0.10
0.12
0.11
0.12
0.10
0.10
8.41
7.93
0.16
7.99
7.93
8.11
8.67
8.95
9.25
9.43
0.09
9.63
11.59
3.21
12.44
12.21
11.97
11.14
11.53
11.72
11.83
12.47
12.49
2.60
2.87
3.27
3.03
2.72
2.70
2.47
2.48
0.05
0.11
2.68
0.10
0.56
0.36
0.29
0.29
0.11
0.11
0.23
0.17
0.16
0.16
0.06
0.08
0.06
0.08
0.06
0.08
0.06
0.06
0.06
0.05
0.04
0.05
0.05
0.06
0.06
0.05
99.53 100.49 100.82
101.05
100.38
101.31
101.15
100.93
101.56
100.93
101.35
101.48
2.84
2.67
3.25
3,08
2.97
3.06
2.94
3.02
3.15
8.50
8.63
7.94
8.38
9.63
10.19
9.54
9.98
10.33
6.54
6.36
10.49
3.43
10.53
0.27
0.26
0.19
0.21
0.20
0.14
Cr203
0.04
0.03
0.04
Total
99.86
CaO
Na20
K20
3.38
12.32
2.54
0.07
Na8
2.89
2.77
2.94
Fe8
7.70
7.40
8.24
2.76
9.43
CaO/Al203
0.71
0.71
0.76
0.79
0.78
0.78
0.63
0.68
0.68
0.68
0.73
0.72
0.71
K20/Ti02
0.13
0.13
0.12
0.04
0.08
0,07
0.38
0.24
0.25
0.25
0.06
0.07
0.07
Table 6 (cont'd)
Dredge
Longitude (°E)
Latitude (°S)
Depth
# Analyses
Type
Si02
Ti02
Al203
FeO
MnO
MgO
CaO
Na20
K20
Cr203
Total
50.19
D138.3
112.86
50.19
3842
3845
3845
3845
40
n = 29
n=3
n= 1
N
N
50.47
E
50.86
N
50.76
50.89
2.34
1.46
D134.2
D135.1
D135.2
D136
D138.1
112.49
112.59
50.31
112.86
50.30
112.59
50.31
112.69
50.21
112.32
50.32
50.32
3600 3327.5
3483.5
3650
3650
3747.5
3747.5
n = 35
n=6
n=41
n=1
n=16
n= I
N
51.12
D131
D132
D133
111.78
49.78
111.13
49.78
111.34
50.06
3520
3520
n=4
n = 14
111.13
D138.2
112.86
50.19
D134.1
112.49
50.30
D130.2
D130.1
n = 17
n
50.76
N
51.12
N
51.04
N
50.69
N
50.94
51.67
N
51.25
1.55
1.31
1.29
1.49
1.44
1.46
1.47
1.78
1.67
1.56
2.30
15.85
16.43
15.32
15.52
15.58
15.34
15.65
14.84
15.20
15.31
14.43
14.38
15.25
10.82
8.93
N
N
51.33
9.08
8.61
9.34
8.76
8.92
9.38
0.14
0.13
0.14
0.15
0.15
0.14
7.68
8.15
8.07
7.61
8.16
8.30
11.33
11,47
12.43
11.95
12.16
11.73
N
9.66
0.13
10.07
9.73
9.47
10.94
0.15
0.16
0.16
0.16
0.16
0.13
7.98
11.97
7.72
8.27
7.99
7.16
6.82
7.71
11.24
11.43
11.60
10.48
10.55
12.33
2.68
2.92
3.01
2.84
3.07
3.05
2.86
0.10
0.12
0.13
0.19
0.20
0.23
0.14
0.12
0.13
0.21
0.20
0.15
0.05
0.08
0.04
0.04
0.07
3.19
3.05
2.61
3.20
2.93
0.10
0.09
0.17
0.08
0.12
0.10
0.08
0.09
2.70
0.10
0.13
0.06
0.06
0.06
0.12
0.05
0.12
0.05
0.12
0.05
0.06
0.14
0.04
100.16 100.56
100.06
100.29
100.27
101.49
100.26
100.91
100.11
99.75
99.04
99.99
2.82
9.88
2.68
2.82
3.11
2.84
2.76
2.62
2.75
9.63
9.60
10.18
9.45
9.55
8.86
8.45
0.76
0.07
0.76
0.76
0.75
0.76
0.73
0.73
0.81
0.07
0.07
0.08
0.07
0.08
0.08
0.16
100.41
Na8
3.07
3.11
2.64
3.05
3.00
Fe8
8.55
8.85
9.46
8.11
9.19
CaOIAI2O3
0.71
0.70
0.81
0.77
0.78
K20/Ti02
0.06
0.07
0.06
0.11
0.06
Table 6 (cont'd)
Dredge
Longitude (°E)
Latitude (°S)
Depth
# Analyses
Type
Si02
Ti02
D138.4
112.86
50.19
D139
D140.1
D140.2
112.86 113.11
50.19 50.25
113.45
113.45
50.30
D138.5
50.30
D142
D143.1
D143.2
D143.3
D144.1
D144.2
D144.3
113.62 114.48
50.35 49.86
114.95
114.95
114.95
115.21
115.21
115.21
49.95
49.95
49.95
50.01
50.01
50.01
D141
3845
3845
3614
3320
3320
3001.5
4300
4090
4090
4090
3997
3997
3997
n=1
n=5 n = 10
n=7
n=1
ii = 22
n=2
n=5
n=2
n=1
n=5
n=2
n=4
N
50.92
N
50.50
N
51.33
E
51.38
N
N
51.34
N
51.44
N
N
51.72
50.59
N
50.15
N
50.75
N
51.04
N
50.49
51.34
1.55
1.45
1.38
1.90
1.93
1.84
1.50
1.36
1.46
1.54
1.35
1.21
1.52
Al203
14.78
15.12
15.30
14.11
14.27
14.68
15.92
16.24
15.95
15.55
15.43
15.78
15.32
FeO
MnO
MgO
9.50
9.31
8.52
10.53
10.70
9.97
8.65
8.18
8.50
8.67
8.35
7.88
8.70
0.16
0.16
0.12
0.14
0.14
0.12
0.10
0.11
0.12
7.93
7.81
6.72
6.48
7.85
0.10
8.36
0.11
7.51
0.16
6.94
8.03
7.50
8.06
8.57
0.12
7.47
12.17
12.03
11.86
11.02
11.12
11.33
11.22
11.48
11.29
2.69
2.85
3.20
3.26
3.26
3.09
2.95
2.88
0.18
0.13
0.22
0.23
0.27
0.16
0.17
0.19
0.14
0.10
0.14
0.11
0.12
0.14
0.12
2.84
0.16
11.49
2.82
10.93
2.85
11.19
3.01
11.5 1
0.18
0.19
0.18
0.15
0.13
0.13
0.05
0.16
0.07
0.10
0.05
0.09
0.06
0.05
0.13
0.04
CaO
Na20
K20
3.30
Cr203
0.05
0.05
0.06
0.02
0.01
0.02
0.05
Total
99.91
99.91
98.65
98.55
99.66
100.03
100.05
100.25
100.04
99.96
98.36
98.27
98.42
Na3
2.67
2.66
2.79
2.73
2.70
2.87
3.04
2.98
3.03
2.77
2.91
3.04
3.11
Fe8
8.69
9.19
8.20
8.41
8.18
8.21
8.41
8.79
8.56
7.84
8.46
8.82
7.82
0.70
0.11
0.74
0.73
0.73
0.71
0.12
0.10
0.08
0.09
CaO/Al203
K20/Ti02
0.82
0,12
0.80
0.78
0.78
0.78
0.77
0.70
0.71
0.09
0.10
0.12
0.12
0.15
0.10
0.11
Table 6 (cont'd)
wc-45
95.18
42.51
wc-43
90.69
42.84
45.00
wc-46
95.77
46.17
2335
2537
2384
2872
3231
n=1
n=1
n=1
n=1
n=1
n=1
N
51.27
N
51.37
N
51.07
N
50.31
N
50.74
N
50.98
N
51.28
1.82
1.79
1.83
1.74
1.61
1.29
1.71
14.32
14.52
14.48
14.43
14.57
14.88
15.87
14.94
7.55
10.45
10.56
10.64
10.64
10.30
9.81
8.49
0.10
7.17
0.11
0.14
0.10
0.13
0.15
0.13
0.12
0.11
7.94
6.57
7.32
7.38
7.34
7.59
7.57
8.55
9.89
0.10
7.60
wc-42
90.08
41.69
wc-40-3
88.69
41.69
2335
2335
n=9
n=1
1.21
N
51.07
2.26
16.01
15.97
7.65
7.95
D145.4 D146 D147
116.72 117,18 117.52
49.27 49.51
49.70
wc-40-1
88.69
wc-40-2
88.69
41.69
4632.5
n=9
51.41
N
52.30
1.35
16.40
7.53
0.08
D145.1
D145.2
D145.3
116.72
116.72
49.27
116.72
49.27
4665
4665
4665
4665
4817.5
n=6
n=1
n=2
n=1
E
51.10
E
E
51.87
E
51.45
1.27
1.18
1.22
Al203
16.81
16.64
FeO
MnO
MgO
CaO
Na20
K20
P205
Cr203
7.11
0.09
Dredge
Longitude(°E)
Latitude (°S)
Depth
#Analyses
Type
Si02
Ti02
49.27
7.65
7.66
0.10
7.43
10.43
10.82
10.79
10.77
11.14
10.08
11.77
11.78
11.76
11.61
11.89
11.74
11.34
3.71
3.79
3.80
3.84
3.27
3.35
2.80
2.79
2.81
2.61
2.90
2.73
0.43
0.21
0.22
0.12
0.21
0.12
0.12
0.12
0.13
0.11
0.09
0.17
0.20
0.15
2.83
0.10
0.15
0.17
0.11
0.21
0,14
0.15
0.14
0.15
0.11
0.10
0.12
0.04
0.05
0.06
0.05
0.05
0.02
0.05
0.04
0.04
0.05
0.06
0.06
0.03
98.80
99.97
99.24
99.03
99.76
98.70
100.47
100.67
100.33
99.19
99.81
100.02
99.92
Na8
3.58
3.66
3.59
3.53
3.25
2.82
2.55
2.56
2.57
2.46
2.74
2.94
2.68
Fe8
6.53
6.96
6.71
6.57
7.44
8.08
9.43
9.60
9.55
9.62
9.09
9.41
9.23
CaO/Al203
0.62
0.34
0.65
0.66
0.67
0.70
0.81
0.81
0.81
0.74
0.76
0.17
0.17
0.10
0.07
0.07
0.06
0.80
0.08
0.80
0.17
0.70
0.09
0.07
0.07
0.06
Total
K20/Ti02
Table 6 (cont'd)
Dredge
Longitude(°E)
wc-47 wc-48 wc-49 wc-5O wc-51 wc-52 wc-53 wc-54 wc-55 wc-56 wc-57 wc-58 wc-59 wc-61 wc-62 wc-65 wc-66
95.84
95.93
95.97
96.06
97.48
98.84
98.97
99.05 104.17 104.28 105.26 107.14 108.66 108.92 110.60 113.72 117.84
Latitude(°S)
46,04
46.09
46.14
46.19
47.43
48.06
48.10
48.02
48.17
48.21
48.98
48.31
49.13
49.40
49.88
50.38
49.78
3286
2990
3126
3184
2422
2616
2650
2584
3552
3495
3664
2930
3850
3460
3074
4065
n=1
n=1
n=1
n=1
n=l
n=i
n=l n=1 n-i n=1 n=1
n=6 ni n=1
3870
n=i
n=1
n=1
N
N
51.49
N
51.75
E
50.25
51.88
N
50.60
51.02
51.31
50.55
N
51.34
N
51.74
50.79
N
50.85
N
51.72
N
52.16
N
50.67
N
50.04
N
50.34
1.65
1.62
1.37
1.47
1.87
2.14
1.65
2.03
1.91
1.75
1.62
1.71
1.32
1.23
1.44
1.71
1.20
Al203
15.11
17.36
14.87
15.24
13.91
13.69
15.21
14.79
14.35
14.44
14.49
14.48
15.16
15.96
14.74
14.40
15.13
FeO
MnO
MgO
CaO
Na20
K20
9.65
7.81
9.08
9.20
10.74
11.67
10.07
10.46
10.21
10.21
9.62
10.10
9.13
8.32
8.70
9.44
8.47
0.09
0.09
0.11
0.15
0.13
0.13
0.11
0.13
0.12
0.12
0.10
0.13
0.11
0.12
0.13
0.13
0.12
8.23
8.22
7.86
8.16
12.02
2.75
Depth
#Analyses
Type
Si02
Ti02
Cr203
Total
NN,SMN,SM
N
7.65
7.79
8.01
8.00
7.03
6.46
7.39
7.19
7.31
7.53
7.37
6.70
11.67
11.33
12.27
11.78
11.11
10.46
11.14
10.92
10.61
10.76
2.66
2.92
2.75
0.15
0.22
0.04
0.06
0.17
0.03
0.18
0.01
0.15
0.05
0.10
0.04
0.04
0.20
0.04
0.16
0.17
0.04
2.80
0.16
2.71
0.09
2.95
0.20
2.93
0.66
0.22
2.72
0.16
2.89
0.09
0.12
2.66
0.13
11.88
2.58
11.39
3.05
10.97
3.34
12.09
2.79
11.20
2.77
7.20
11.43
3.08
100.61 100.22 100.14 100.67
98.48
98.90 100.12
99.45
0.11
0.15
0.19
0.07
0.25
0.14
0.16
0.07
0.15
0.09
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0.15
0.10
0.17
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99.13
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98.66 100.31 101.12
97.93
97.89
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2.78
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8.73
2.98
2.67
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Feg
2.66
9.07
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9.50
0.77
0.65
0.77
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0.76
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0.73
9.11
CaO/Al203
9.10
0.82
2.79
8.68
0.74
0.74
0.74
0.77
0.76
0.78
0.76
0.77
0.79
0.79
K2OITiO2
0.05
0.41
0.07
0.09
0.08
0.09
0.09
0.11
0.08
0.09
0.09
0.11
0.06
0.12
0.08
0.14
0.08
Na8
115
counted for 20 seconds, while P and K were counted for 30 seconds and Cr for 60
seconds. Typical standard deviations ranged from 0.01 oxide wt. % for K20 and Cr203
up to 0.12 oxide wt. % for Si02.
Sample Information
A total of 78 dredges and 22 rock cores were successfully completed (Fig 21).
The sampling density varied widely, from 1 to 12 dredges (including seamounts) per
segment. Lavas from these 110 localities were divided into 181 chemical groups, where
each group represents lavas of similar composition from the same location. Lavas within
each group were averaged to obtain a group composition. This allows a more
straightforward comparison of variations along axis because it is not biased by numerous
analyses of similar samples at individual locations. These group compositions are used to
represent the data in all further discussions.
Na8 Calculation
Nag represents a back-calculation performed on a fractionated lava to determine
what its Na20 value was at MgO = 8 wt %, before significant crystal fractionation
occured. It is determined by back-calculating down the modeled fractional crystallization
liquid line of descent (LLD) of a suite of lavas to estimate the supposed Na20
concentration at 8 wt % MgO. K+L (1987) first presented this parameter and used a
regression through the global dataset based on segment and regional averages to
determine the equation published in their manuscript. However, Nag can be calculated
other ways. One could use the regional dataset, such as all the data from a single ridge,
or even individual segment data to calculate the most accurate Nag value. For this paper,
Nag values were calculated two different ways, using the global equation published in
K+L (1987) and using a linear regression through the data, segment by segment. Because
116
the calculations yielded similar results, we have reported and plotted only the K+L
approach in order to test the degree to which the SEIR behaved as a proxy for the
calculations following the global ridge system.
Results
Most of the lavas are aphyric in hand sample, although a few contained up to 10%
crystals. Their mineralogy ranges from plagioclase to plagioclase + olivine
clinopyroxene, but plagioclase is the most commonly observed mineral, olivine is found
in about half of the phyric samples, and clinopyroxene is rare. The chemical groups
include 155 N-MORB (K20/Ti02
0.15), 26 E-MORB (K20/Ti02? 0.15), and 1 FeTi
basalt (FeO*> 12 wt %, Ti02 > 2 wt % after Melson, 1976), from 154 axial and 27
seamount locations (Table 6). The number of compositional groups per segment on the
SEIR ranges from 2 to 22, or approximately one per 14 km of ridge length.
N-MORB are the principle composition recovered in the study area. Overall,
these lavas form tight trends wherein the compatible elements decrease, and the
incompatible
elements increase with decreasing MgO. Glass compositions range from 6.2-9.8 wt %
MgO, and vary in degree of crystallization from 9-68%,based on crystallization
calculations in the methods section (Fig 22). The most primitive lavas are found at the
eastern end of the study region, in the deepest axial segments; whereas the most evolved
lavas occur on the shallowest segments and at propagating rift tips. Despite this rough
correlation between lava composition and location, both primitive and evolved samples
erupt throughout the area.
Although the sampling density is of moderate resolution, there are hints of
cyclical patterns along axis in major element compositions. For instance, at 88.92E,
99.l2E, and 1 10.41E, Ti02, FeO*, Si02, and Na2O all exhibit regional lows, while
Al203 shows a regional high. Other elements also vary widely in concentration (Fig 23,
Table 6).
117
Figure 22 Equilibrium crystallization liquid line of descent models for CaO, K20, Na20
and Ti02 versus MgO calculated using the model of Ariskin et al. (1993). Each symbol
along the line represents one percent crystallization. The black cross is x kb, the gray
cross is x kb, and the gray circle is x kb equilibrium crystallization. Note that the 2 and 3
kb models most accurately reproduce the range and magnitude of the data, but that the
higher values of K20 and Na20 cannot be reproduced by any of the models.
Figure 23 (a) Major and minor element variation of the Southeast Indian Ridge lavas
with axial depth and longitude. Black symbols represent N-MORB, while gray symbols
represent E-MORB. Axial samples are denoted by filled circles, seamount samples by
open triangles. Note the strong correlaton between CaO and depth and longitude, and the
nearly invariant behavior of K20 and Ti02 along axis. Also note that there is no pattern
in the distribution of E-MORB along the ridge axis. (b) Na8, Fe8, and CaO/Al203
variations with axial depth and longitude, calculated according to the equations in Klein
and Langmuir (1987). Note the strong correlations in each of the parameters.
118
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121
K20, P205, Na20, Si02, K20/Ti02, Nag and Sig all increase slightly with
increasing axial depth, while CaO, FeO, Feg and CaO/Al203 all decrease (Fig 23). MgO,
Al203, Ti02 and the calculated % crystallization do not vary systematically with depth.
(Fig 23, 24).
Approximately 15% of the recovered lavas are E-MORB (K20/Ti02O.15), and
they were dredged from more than half of the segments (Fig 23). Generally, they are
located either at the western end of first-order segments and/or near propagating rift tips,
but they are always found on short, second order ridge segments near ridge offsets.
The two FeTi basalts were dredged from the far western end of the study area. The first
is located on segment Cl 7, adjacent to the transform fault at 88°E. The second is located
at the boundary between segments C16 and C15, on the southern arm of an overlapping
spreading center.
In detail, each segment of the SEIR shows its own unique behavior. On segments
such as Cl 7, all of the data fall within a single linear array in variation diagrams versus
MgO. However, within that array, all of the data from a single dredge display an oblique
linear array, and each array is offset from the others (Fig 25). On other segments, the
data fall into multiple groups or show no trend at all. In segment C14, the propagating
rift is chemically distinct from the failed rift.
Individual segments also distinguish themselves in their distinct Na8 and Fe8
behaviors. In Figure 26, Na8 is plotted versus Fe8 with each segment in a different
symbol in order to see these distinctions. Based on the Klein and Langmuir (1987)
model, it is thought that the areas with the highest Na8 and the lowest Fe8 are underlain
by colder mantle, and that those with the lowest Na8 and highest Fe8 are underlain by the
hotter mantle. Within this framework, the global array spans the spectrum from hot to
cold mantle. Individual segments of the Southeast Indian Ridge data also exhibit a range
of Na8 and Fe8 values in the form of a non-uniform progression from high Na8 and low
Fe8 in the east (Segment C2) to low Na8 and high Fe8 in the west (Segment C 17).
However, the data from individual segments form trends that are oblique to the overall
SEIR dataset, as well as the global array, and subparallel to one another (Fig 26). As a
whole, the SEIR dataset and the global array show a progression from shallow melting in
122
Figure 24 Comparison of Southeast Indian Ridge with Global Array in Na8, Fe8, and
CaO/Al203 versus depth. Note that the SEIR data represents approximately 50% of the
global range, yet has a shallower slope than the global array. Symbols are the same as in
Fig. 23.
123
Figure 24
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0.85
0.80
0.70
0.65
0.60
0.55
0
1000
2000
3000
Depth (m)
4000
5000
6000
124
Figure 25 Ti02 versus MgO for segments Cl 7 and C14 of the Southeast Indian Ridge.
Note that individual dredges form trends oblique to the overall segment trend and that the
propagating rift tip dredges are distinct from those away from the rift tip.
3.0
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125
Figure 26. SEIR Nag versus Fe8 with different symbols for each segment. Segments
generally progress from higher Nag and lower Fe8 in the east to lower Nag and higher Feg
in the west, suggestive of higher pressures and extents of melting in the west. Individual
segments exhibit trends oblique to the Global Array and one another. Some segments are
virtually invariant in Nag. Klein et al. data published in Klein et al. (1991). Pyle data
published in Pyle (1994).
126
Figure 26
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135
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Depth (m)
4000
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127
the east to deeper melting in the west, as evidenced by the instantaneous fractional
melting trajectories overlain on Fig 26, after Langmuir et al. (1992).
Discussion
Crystallization Models
We performed fractional and equilibrium crystallization models at multiple
pressures in order to determine the type of crystallization, as well as the pressure and
extent to which it occurred. Using the program developed by Ariskin et al. (1993), we
determined that the 2-3 kb equilibrium crystallization models most accurately reproduce
both the overall range of values and the inflections of the SEIR dataset (Fig 22).
However, even the best-fitting models could not reproduce the large scatter within the
data or the higher concentrations of the incompatible elements (overenrichment) at
moderate to high MgO (Fig. 22).
We also used the crystallization models to calculate the degree of crystallization
each lava has experienced during the time between their creation and eruption at the
surface, by determining the amount of crystallization per weight percent decrease in
MgO. From the initial MgO (i.e that which is in equilibrium with the mantle), and its
change during cooling and ascent, we determine the amount of crystallization. To do so,
we used the Ariskin et al. (1993) model to calculate an equilibrium crystallization liquid
line of descent. Since the model does not pass through every single datum, we must
develop an algorithm for the amount of crystallization for any lava. From the model, the
increase in crystallization per unit decrease in MgO can be calculated. For the SEIR, the
lavas crystallize 17% per unit decrease in MgO above 8.6 wt %, and 23% per unit
decrease in MgO below 8.6 wt %. (The inflection point in the equilibrum crystallization
LLD at MgO=8.6 wt % represents the point where clinopyroxene joins the cotectic.) The
amount of crystallization felt by a magma with MgO8.6, is equal to the difference in
128
MgO between 10.36 (the MgO we inferred to be in equilibrium with the mantle) and the
actual MgO content of the magma, multiplied by 17% crystallization. For magmas with
less than 8.6 wt % MgO, the amount of crystallization undergone is equal to the amount
of crystallization between MgO=1 0.36-8.6, plus the amount that takes place below
MgO=8.6. Based on these calculations, the SEIR lavas are best modeled by equilibrium
crystallization ranging from less than 10 to nearly 70%.
Similarities and Differences Between the SEIR and the Global Array
To a first approximation, the Southeast Indian Ridge data comply with the global
model set forth in K+L. The SEIR lava data overlap with the global array and represent
50% of the total global range in Na8, Fe8, and CaOIAl2O3. Regions considered to be
underlain by warmer mantle, such as the west end of the study area, have lower Na8 and
CaOIAl2O3, and higher Fe8 values consistent with clinopyroxene fractionation, higher
extents and pressures of melting and a longer melting column. In contrast, lavas derived
from colder mantle near the Australian-Antarctic Discordance have higher Na8 and
CaOIAl2O8, and lower Fe8 values, consistent with lower extents and pressures of melting,
as well as a shorter melting column. However, within the SEIR data, the scatter is large
and more than one third of the data fall outside the global array field. The magnitude and
range of compositions are similar to those reported in K+L (1987; 1989) for the Indian,
South Atlantic and Southern Ridges, and unlike either the East Pacific Rise or the
northern Mid-Atlantic Ridge. In each case, however, a linear regression through the
SEIR data produces a line with a shallower slope than that of the K+L global array (Table
7). When compared with the 95% confidence intervals for the SEIR regressions, the
Klein and Langmuir global trends are found to be statistically distinct, confirming that the
SEIR lavas behave differently than the global array (Table 7).
In particular, for the plot of Na8 versus Fe8, we can mathematically derive the
slope of the global array from the original Na8 and Fe8 equations published in K+L
(1987) by solving for depth and then setting the two equations equal to one another. This
Table 7. Comparison of linear equations of Na8, Fe8 and CaO/Al203 from previous research and this paper
Error in
Relationship
Equation of Line
Na8-Depth
2.9x10-4 (depth) + 1.60
8.75x10-5 (depth) + 2.49
-6.6x10-4 (depth) + 10.98
-2.30x10-4 (depth) + 9.58
-3.7x10-5 (depth) + 0.86
-2,87x10-5 (depth) + 0.86
-0.22 (depth) + 4.45
-0.126 (depth) + 3.88
Source
Brodholt and Batiza
This paper
Brodholt and Batiza
Fe8-Depth
This paper
CaO/Al203-Depth Brodholt and Batiza
This paper
Brodholt and Batiza
Na8-Fe8
This paper
R
0.87
0.32
0.66
0.27
0.74
0.53
0.65
0.39
Slope
Error in
Intercept
95% Confidence Interval
±2.3x10-5 ±0.07
- 1.33x10-4<slope<4.22x10-5
±7.2x10-5 ±0.222
-8.8x10-5<slope<-3.71x1 0-4
±4x 10-6
±0.0126
-3.7x10-5<slope<-2. lx 10-5
±0.026
±0.23
-0.178<slope<-0.074
130
yields a slope of magnitude 0.224. (It is unclear why this does not yield a negative
slope.) In contrast, a linear regression through the SEIR data reveals a slope of 0.126,
with a 95% confidence interval of 0.178 to 0.074 (Fig 26, Table 7). Since the K+L
slope does not fall within the 95% range, we can conclude that the two slopes are
statistically different.
The effect of the different slopes for any given depth is to yield higher Na8 and
CaO/Al203 values, and lower Feg values than is predicted by the K+L model. It is
therefore inaccurate to use the K+L model to predict values in the SEIR case. For
example, at a depth of 3000 m, the K+L model would predict values of Na8 = 2.4, where
the actual value is Na8 = 2.8. A higher Na8 value suggests that the SEIR lavas are the
products of shallower, less extensive melting than is predicted by the global array.
Similar offsets occur in the other K+L parameters (Table 8).
Table 8. Predicted and actual values of Na8, Fe8 and CaO/Al203 for theSEIR and regional
datasets.
Parameter
Predicted
Actual SEIR only
Actual-ASP-SEIR-AAD
Nag
Fe8
CaOIAl2O3
2.4
2.8
9
0.77
2.6
9.15
0.75
9.8
0.73
Because the study area is only one portion of the Southeast Indian Ridge, it is
important to consider how the region between 88° and 118° fits within the regional
context. In a linear regression through the segment averages of the Amsterdam-St. Paul,
88°-li 8°, and Australian-Antarctic Discordance data, we find that the regional line is
131
Figure 27 ASP, SEIR, and AAD Nag versus longitude and depth. Note that the three
regions exhibit a coherent trend versus both longitude and depth. Also note that each
area forms a trend oblique to the overall trend, but subparallel to one another. The ASP,
SEIR, and AAD data are offset from the global array to higher Nag values.
Figure 26
3.4
-
C2
.
3.2
C3
C4
X:
C7
El
3.0
ri
:: C8
-
z
x
Oii.
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2.8
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-
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-
2.0
7.0
7.5
8.0
8.5
9.0
Fe8
9.5
10.0
10.5
11.0
133
closer to the global array, but still distinct. For the regional data (ASP-SEIR-AAD), the
Nag, Fe8, and CaO/Al203 trends are actually parallel to, yet offset from the global trends
to higher Na8 and CaO/Al203 and lower Fe8 at any given depth (Fig 27; Table 8). In
addition, in Nag versus depth, individually, each group forms a trend oblique to the global
trend and the regional trend, but subparallel to one another.
The Southeast Indian Ridge data also differ from the global array in segment-
scale processes. On the Mid-Atlantic Ridge, K+L observed "local" trends that are
oblique to the global array, yet parallel to one another, which they suggested might
represent the evolution of lavas within a single melting colunm ("intra-column"), as
opposed to those of the global array, which represent the relationship between all of the
melting columns ("inter-column"). Similar to the local trends observed in the MidAtlantic Ridge data, on the Southeast Indian Ridge, individual segments do form oblique
trends that cross-cut the overall trend in the data, as well as the global array. However,
this is not true for all segments, and for the segments that do show trends, they are not
parallel to one another, nor do they mimic the slope of the Atlantic local trends. Many of
the segments exhibit nearly invariant behavior in Na8, similar to trends reported at the
ASP region by Douglas-Priebe (1998). One possible explanation for how the local trends
can be parallel, yet offset, is that each melting column might have a different Na20
content in its starting composition to provide the offset, and then undergo the same
processes to provide the parallel trends. The fact that the segment-scale trends of the
SEIR are not parallel is consistent with each melting column having its own unique
composition and/or magmatic history. However, since some segments, like C2, exhibit
no local trend, but follow the global array instead, some columns, like all of the EPR,
show only the global trend. For segments to have nearly constant Na8, lavas must
undergo a range of mean pressures of melting, but nearly constant extents of melting.
Douglas-Priebe (1998) cited imperfect pooling of melts and mantle heterogeneity as
possible causes of this behavior. Given the fact that neither the East Pacific Rise nor the
Southeast Indian Ridge exhibits the local trends seen on the Mid-Atlantic Ridge, we
cannot consider the intra vs inter-column assessment to be a global phenomenon. To
summarize, although some segments along the Southeast Indian Ridge exhibit localized
134
trends, others do not, indicating that the behavior is non-uniform, and therefore
inconsistent with simple predictions from the global model set forth in K+L. We believe
that the ridge is multi-dimensional and that certain areas may have unique combinations
of parameters. At certain critical or "threshold" values of spreading rate, composition,
scale of mantle heterogeneities, and mantle temperature variations, contrasting and
different patterns may be expressed in erupted lavas that obscure or modify the first firstorder mantle temperature control.
Causes for Deviations from K+L
The chemical compositions erupted on the Southeast Indian Ridge are
inconsistent with the current global paradigm. There are several possible reasons for
these distinctions. First, the global array may not represent the global range of values.
Klein and Langmuir relied strongly on the Mid-Atlantic Ridge and the East Pacific Rise
for the development of their global dataset. Hamelin et al. (1986) found that the Indian
Ocean has remained partially isolated from the other ocean basins, thereby preventing it
from mixing with the global reservoir and therefore, allowing it to retain a distinct
isotopic signature. In addition, the fact that the Indian Ocean is the youngest ocean basin
provides further opportunity for the mantle to be isotopically and chemically distinct, and
adds credence to tl1e possibility that it contains more heterogeneities (possibly at different
scales) than the other oceans because it has had less time to thoroughly mix or is in closer
proximity to them (e.g. continental sources).
Second, the offsets in both Na8 and Feg suggest that either the underlying mantle
temperature is lower andlor the depth of the ridge axis is shallower than the global
paradigm would predict. Lower temperatures would be in accord with the AAD which is
known for its anomalously cold mantle and is the topographically deepest part of the
global mid-ocean ridge system. However, shallow crust would require a mechanism for
crustal inflation unrelated to mantle temperature, such as: proximity to hotspot or
anomalously thin (dynamically compensated) crust, differing flow rates. Because
135
individual segments progress non-uniformly from high Na8 and low Fe8 to low Nag and
high Fe8, either the mantle temperature or crustal thickness variations are non-uniform.
This is consistent with mantle tomography work by Roult et al. (1994) who found non-
linear temperature variations along axis. If this is true, then the mantle beneath the 88E
area is, on average, colder than "normal" mantle underlying ridges elsewhere of this
depth, since it is at the average axial depth of 27OO m.
With regard to the SEIR data,we have failed to find modeled conditions for
shallow level fractionation that predict the observed data. This suggests that either
fractionation is occurring at a higher pressure, or that fractionation is not the only process
at work. K+L also assume that all variability in Na8, outside of error introduced at low
MgO, is the result of mantle processes (differing pressures and extents of melting).
However, there are many other ways to introduce variations in Na. In the mantle, Na is
found in the jadeite component of clinopyroxene. According to Putirka (1998), the Na
content is sensitive to the Al content of the liquid, and, in addition,
DNa
p'q increases
substantially with increasing pressure, ranging from --O.O5 to 1. Pyroxene phase
equilibria and the modes in which pyroxene occurs in the mantle are not well constrained
(Longhi and Bertka 1996; Shimizu et al. 1997). Changes in these parameters would
cause significant differences in Na20, and therefore Na8 values of the derived lavas. In
addition, any mantle heterogeneities could also cause deviations in Na20 and Nag from
the global array.
136
Table 9. Observations and conclusions based on the Southeast Indian Ridge major
element data.
Observations
Conclusions
SEIR Na8, Fe8, CaO/Al203 show
same correlations as the K+L global
dataset, but with different slopes
SEIR Na8, Fe8, CaO/Al203 represent
approximately 50% of the global
range
Areas near ASP have low Na8, high
Fe8, Low Si8
Areas near the AAD have high Na8,
low Fe8, and high Sig
Individual segments show no
systematic trends
Scatter in parameters
Segments and groups of segments
exhibit sub-parallel "local"-type
trends
Local-type trends are not parallel to
one another and not as well-defined
as on MAR
Local trends are invariant in Na8
Segments show a non-uniform
progression from shallow to deep
One segment, C8, shows lower Na8
and higher Fe8, evidence for warmer
than predicted mantle temperature
for this longitude
Chemistry constant across certain
segment boundaries
Chemical differences no more
distinct across transform boundaries
than segment boundaries
1. SEIR is not representative of the global system; 2. K+L
did not incorporate the global network when formulating
the global eqns; 3. AND/OR since Indian Ocean is
younger and isotopically distinct, maybe it is also
chemically distinct with younger, hone
Axial morphology, depth and geophysics represent only a
fraction of the total global range; chemistry is consistent
with other properties
This is consistent with high pressures and extents of
melting, and hot mantle (longer melting column) near
ASP.
This is consistent with low pressures and extents of
melting and colder mantle (shorter melting column) near
AAD.
Sample density is not high enough and either temperature
variations are not systematic OR they are not resolvable at
this scale
May be due to K+L type"local trends"
1. These could be the intracolumn trends described in
K+L; 2. SEIR is more MAR-like than EPR-like because
EPR doesn't show local trends; 3. This would imply the
absence of a long-lived or steady-state magma lens.
Indicates that separate columns are not undergoing
identical processes
Also true for the ASP. Implies that the same extents of
melting are occuring at different pressures.
1. whatever variations are causing the depth gradient are
non-monotonic; 2. crustal thickness variations would be
due to different extents of melting due to differences in
mantle temperaturemaybe deviations from uniform
progression represent magma inject
Magma injection?
1. Combine segments CS and C6, C9 and C 10, and C 13
and C 13/14; separate C8/9 from its neighbors; 2. Tectonic
boundaries may be unrelated to changes in mantle
temperature
1. The tectonic corridors described by Hayes and Kane
(1994) do not translate into major element or K+L
parameter distinctions; 2. The chemical cell is not obvious
and must be determined by looking at other parameters,
either chemical or otherwise.
137
Conclusions
In summary, the Southeast Indian Ridge provides a unique opportunity to study
the controls on crustal accretion in the absence of variations in spreading rate. In this
study, we have found that the SEIR dataset does not comply with the commonly accepted
paradigm of differences in morphology and chemical composition being the result of
changes in the spreading rate. Specifically, we have found that:
The SEIR exhibits approximately 50% of the global range of MORB compositions in
Nag, Fe8 and CaO/Al203.
The maj or element chemistry of the erupted lavas on the SEIR is consistent with other
properties of the ridge, such as variations in depth and morphology, which also
exhibit large ranges.
The SEIR lavas exhibit a range in MgO, but none of the lavas is primitive enough to
be in equilibrium with the mantle. Therefore, the most primitve melts must not erupt
at the surface.
Lavas are the products of large extents of crystallization based on equilibrium
crystallization calculations performed using the Ariskin et al. (1993) model.
The large range of Na8 and Fe8 values suggests that the SEIR lavas were created by
large range of pressures and extents of melting.
Since the SEIR lavas progress from higher Na8 and lower Fe8 in the east to lower
Na8 and higher Fe8 in the west, we agree that there must be a temperature gradient
along axis. However, becuase the progression is non-uniform, the temperature
variations must also be non-uniform. It appears that the mantle is colder under 88°E
on the Southeast Indian Ridge than would normally be found elsewhere.
It is possible that the major element mantle heterogeneity is stronger in the Indian
Ocean than in other ocean basins.
138
Acknow1edements
We thank Brendan Sylvander and Frank Sprtel for collecting the major element
data published in this paper.
139
Summary
Rachel Sours-Page
July, 2000, 2 pages
140
The study of mid-ocean ridge basalts is all about magmatic processes. The
chemical composition of MORB, their crystals and inclusions allows us to distinguish
between processes and determine the unique history of a single lava. Based on the three
studies presented here, it is evident that MORB lava compositions reflect extensive
magmatic processing. In all cases, lavas have undergone significant pooling and
crystallization that have modified their original chemical compositions.
On the Juan de Fuca Ridge, we found that the two coeval lava suites could not
have been formed from the same mantle source. Instead, there must be localized mantle
heterogeneities that provide both a refractory harzburgite and a relatively enriched
clinopyroxenite in close proximity. In addition, the study of melt inclusions in that
region proved that melt inclusions and their associated lavas are genetically related and
that, at least in some regions, melts pass through the melting regime as discrete packets
and do not mix with one another until reaching the base of the crust.
In contrast, on the northern East Pacific Rise, we found that there are fewer
complexities in the mantle, but more in the crust and axial magma chamber. Axial and
seamount samples share the same mantle source, but the seamounts experience the axial
magma chamber to a lesser degree, causing systematic differences in the two magma
groups in terms of degree of fractionation and crystal content. In addition, we found that
within the crust, both groups experience crystal sorting, wherein the crystal are removed
from the lava. And, for both groups, fractionation must preceed magma mixing.
From the Southeast Indian Ridge study, we found that, whatever processes are
taking place, they control not only the chemistry of the lavas, but also the morphology
and geophysics of the ridge axis. Lavas of the SEIR have undergone significant
fractionation and crystallization, however, the dominant control on lava chemistry is the
deep-seated mantle temperature gradient, which effects the pressures and extents of
melting, modifying the character of the lavas systematically from one end of the study
area to the other. We found that the processes taking place on the SEIR are inducing the
same variations at constant spreading rate that others usually attribute to varying
spreading rates. We conclude that mid-ocean ridge processes are not spreading rate
reliant.
141
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