A Field, Petrologic, and Geochemical Study of
the Callahan Lava Flow, a Basaltic Andesite
from Medicine Lake Shield Volcano, California.
by
Rosamond Joyce Kinzler
S.B.
Massachusetts Institute of Technology
(1984)
Submitted to the Department of Earth, Atmospheric
and Planetary Sciences in Partial Fulfillment of
the Requirements of the Degree of
Master of Science
at the
Massachusetts Institute of Technology
May, 1985
The author hereby grants to M.I.T. permission to
reproduce and distribute copies of this thesis in
whole or in part.
Signature of AuthorDepartment of Earth, Atmospheric,
and Planetary Sciences, May 24, 1985
Certified by
J
Accepted by
l.m
--
j
........
Dr.T.L. ,Grove, Thesis Supervisor
Theodore Madden, Chairman, Department
Committee on Graduate Students
MASSACHUSETTS INSTITUTE
OF TECHNOLOGY
MAY 31 1985
LIBRARIES
Lindgren
Acknowledgements
I'd like to thank Dr. T.L. Grove for his advice and support, especially for his challenging comments on the rough draft.
Thanks also to the many graduates at MIT who provided advice, encouragement, support, and many a 12th floor smile; Micheal Baker,
Daniel Tormey, Tanya Furman, Jane Selverstone, Dave Gerlach, Don
Hickmott (who saved the day by saving my runoff files), and my office mates: Tom Juster, Alan Leinbach and Sung Yan. Special
thanks to the isotope people, Levent Gulen and Vincent Salters, for
their very patient help.
Daniel Orange, my fellow master's student, provided much
support through our "master's thesis support group", thank you Danny.
Carl Bespolka deserves special mention for not only putting
up with me over the past 6 months, but making it bearable besides.
I'd like to thank my mother, Kennen Kinzler, who truly knows how to
say the right thing at the right time, and the rest of my family for
their loving support.
I dedicate this thesis to my brother Karl, who believed in
me in a way that will remain with me always.
Abstract
The basaltic andesite Callahan flow at Medicine Lake volcano, California, exhibits variations in major elements, trace
elements and radiogenic isotopic ratios. Two compositionally distinct plagioclase phenocryst populations occur in the flow, and
disequilibrium textures are common. This study investigates the
chemical variations and textural relations in terms of their petrologic origin. Four models are applied to the data; 1) partial
melting of some combination of the subducted oceanic plate and the
hydrated mantle above, 2) fractional crystallization from a basaltic
parent, 3) combined fractional crystallization and assimilation, and
4) mixing between basaltic and rhyolitic magmas. The major element
variations are best explained by a combined assimilation and fractional crystallization model (AFC), in which the ratio of rate of
The
assimilation to rate of fractional crystallization is 1.5:1 (r).
consistent
are
Zr)
and
Ba,
incompatible trace element variations (Rb,
with r values greater than 1:1. The lack of distinct variation in
87/86 Sr ratios suggests that the assimilant closely resembled the
parental liquid in terms of 87/86 Sr ratios. The (r) value suggested
by the model is higher than would be predicted by simple thermal models of the AFC process at upper crustal conditions. Modeling the
AFC process in the compositional, thermal boundary layer of a basaltic
magma chamber permits a higher rate of assimilation.
Table of Contents
1)
Introduction .................................
.8
2)
Geologic History...........................
.9
3)
Analytical Methods .............................
.14
4)
Mineral Chemistry and Petrography ............
.17
5)
Major Element Chemistry..........................
.39
6)
Trace Element and Isotope Study .............
.42
7)
Discussion...................................
.50
Partial Melting .........................
.50
Fractional Crystallization ..............
.51
Combined Assimilation and Fractional crys
allization (AFC) .........................
64
Mixing of Basaltic and Rhyolitic magmas..
76
8)
Implications of the AFC model ...............
85
9)
Conclusion .............................
95
10) References ...................................
97
List of Tables
1)
Comparison of Standard Analyses.....................15
2)
Major Elements Analyses .................................. 18
3)
Phenocryst Assemblages ...................................
4)
Electron Microprobe Analyses of Phenocryst Phases ........ 21
5)
Major Element Analyses of Silicic Material from within
and Around the Callahan Flow ............................. 40
6)
Trace Element Analyses ................................... 44
7)
(87/86) Sr Ratios.......................................
45
8)
Mg #s for Selected Callahan samples ..........................
59
9)
Parameters of the Best Fit Major Element AFC model.......68
20
10) Parameters of the Best Fit Trace Element AFC model........73
_ _11_11_^1__ ~11__
1_1_
List of Figures
1)
a.
Geographic Location Map for Medicine Lake ............. 10
b.
Schematic Geology of Medicine Lake Volcano............10
2)
Sample Location Map..................................... 13
3)
Representative Plagioclase Microprobe Analyses............25
4)
a.
Photomicrograph of a Reacted Plagioclase .............. 27
b.
Microprobe Traverse of Same Grain.....................27
a.
Photomicrograph of a Ca-rich Plagioclase .............. 29
b.
Microprobe Traverse of Same Grain ..................... 29
5)
6)
Representative Olivine and Pyroxene Microprobe Analyses...30
7)
a.
Photomicrograph of Mg-rich Olivine and Ca-rich Plagioclase ................................................. 32
b.
Microprobe Traverse of the Olivine Pictured in a......32
C.
Microprobe Traverse of the Plagioclase Pictured in a..33
8)
Photomicrograph of Intergrown Olivine and Plagioclase.....34
9)
a.
Photomicrograph of an Orthopyroxene from 83-31.........37
b.
Photomicrograph of an Orthopyroxene from 83-23a.......37
10) Major Element Oxides Plotted Against wt. % Mg0............41
11) Trace Elements Plotted Against wt. % MgO .................. 46
12) Chondrite Normalized Rare Earth Element Plot .............. 48
13) (87/86) Sr Variation vs. ppm Sr ...............
........ 48
14) Oliv-Cpx-Qtz Pseudo-ternary Diagram with 1 atm. Cotectics,
Peritectics, and an Equilibrium Crystallization Path.......53
15) Oliv-Cpx-Qtz Pseudo-ternary Diagram with High Pressure Cotectics, Peritectics and a Fractional Crystallization
Path ...................................................... 55
List of Figures (cont.)
16) Fractional Crystallization Model ..................... .....
63
17) Combined Assimilation and Fractional Crystallization (AFC)
Model for Major Elements.............................. ..... 69
18) AFC Model for Incompatible Trace Elements ............ ..... 72
19) Mixing Model I.......................................................81
20)
Mixing Model II.....................................................83
21) Sketch of Assimilating, Fractionating Basaltic Magma
Chamber ..........................................................
90
Introduction
The Callahan flow is a
Medicine
Lake
Highland
basaltic
in
andesite
lava
flow
northern California (Fig.
from
la).
It
covers an area of approximately 23 square kilometers, averages 30
to 40 m in thickness, and has a volume of approximately 0.8 cubic
kilometers.
exhibits
The flow is Holocene in age (> 1100 years
variations
in
major
elements,
radiogenic isotopic ratios.
The purpose
understand
which
the
processes
of
Callahan
flow
consists
of
a
which
combined
to
elements,
and
study
to
observed
is
produce
in
the
to
the
flow.
series of eruptions closely
associated in space and time and provides a
process(es)
It
this
contributed
geochemical and petrological variations
The
minor
bp).
generate
"snap-shot"
of
the
the basaltic andesite
stage of the calc-alkaline series lavas represented
at
Medicine
Lake Highland.
The calc-alkaline trend is characterized by Si02, Na20,
K20
enrichment
and
FeO+Fe203,
calc-alkaline
of
the
work
MgO
depletion
has been extensively studied.
series,
and Ringwood (1968), Boettcher (1973)
reviews
and
that
has
and
Gill
(1981)
partial
crust,
(2)
melting
fractional
of
Green
provide
been done on andesite genesis.
Petrogenetic models for andesite genesis fall into 4
(1)
with
Andesite, the intermediate member of
increasing differentiation.
the
CaO
and
hydrated
mantle
crystallization
of
categories:
or subducted oceanic
primary
basaltic
material,
(3)
assimilation
of
a crustal component by parental
basaltic magma and (4) mixing of basalt and melted sialic
My working hypothesis
fractional
is
crystallization
region cannot
petrologic
adequately
that
or
partial
explain
relationships
simple
the
observed
processes
melting
chemical
in
the
crust.
such
as
of some source
variation
and
Callahan flow.
The
evaluation of this hypothesis begins with the characterization of
the
chemical
variation
presenting the
presented
data
above
and
petrological relationships.
collected,
will
be
the
tested
four
After
petrogenetic
against it.
models
The petrogenetic
models will be evaluated in terms of their geological feasibility
and
the
extent
to
which
they accurately predict the chemical
variations and petrological relationships observed in
I
will
include
a
brief
geologic
by
summary
history
of
the
flow.
Medicine
Lake
Highland,
followed
employed,
then the field relations, petrography, compositions of
a
of
the
analytical
methods
phenocryst assemblages, major element, trace element and
study.
In
the
discussion section, the petrogenetic models are
applied to this data, and
constructed.
explored in the
isotope
The
the
implications
next
conclusions.
Geologic History
section,
so-called
of
this
followed
"best
fit"
best
fit
by
a
model
model
summary
of
is
are
my
Fig. 1 a. Geographic location map for
Medicine Lake Highland from Anderson
(1941).
Fig. 1 b. Schematic geology of Medicine Lake
volcano after Anderson (1941). Abbreviations:
a.t.=andesite tuff, m.b.=modoc basalt, pa=platy
andesite, d.=dacite, opoa=older platy olivine andesite, rhyo.=rhyolite, comp. rhy-dac=composite
rhyolite and dacite flows.
o
11.
The Medicine Lake Highland is a volcanic center to the
of
the
High
Cascades
in Northern California.
Previous field,
petrologic and geochemical studies of the Medicine Lake
and
surrounding
areas
include
al.
(1981), Grove et al.,
Highland
those of Peacock (1931), Powers
(1932), Anderson (1941), Mertzman
et
east
(1977a,b,1981),
(1982).
Donelly-Nolan
The Medicine Lake shield
volcano formed during the late Pleistocene (less than or equal to
500,000 years b.p.) on a volcanic plateau consisting primarily of
Warner high alumina basalt.
at
Medicine
Lake
is
A simplified sketch of
provided in Fig.
lb.
years b.p.
geology
Mertzman (1977a,b)
dates the onset of the shield volcano building
900,000
the
at
approximately
with the initiation of the outpouring of the
older platy olivine andesite.
diameter developed.
A shield volcano
some
40
km
in
The shield forming stage terminated with the
eruption of platy andesite at the summit, followed by later platy
olivine
andesite,
Hoffman rhyolite.
eruption
of
Lake
high
basalt
(HAB),
and
Mt.
The most recent eruptive cycle began with
the
Modoc
intersertal-subophitic HAB from vents on the
flanks of the volcanic shield.
andesite
alumina
followed.
Flank eruptions of Modoc basaltic
The Callahan flow formed during this stage.
Eruption of rhyolite and mixed dacite
accompanied
extrusion
of
Modoc HAB and basaltic andesite during this latest eruptive cycle
which terminated approximately 1100 years ago (Heiken, 1978).
The nature and origin of the caldera or depression on top of
the
Medicine
Lake
shield
volcano is unclear.
Anderson (1941)
12.
mapped an andesite tuffaceous unit which he correlated
with
the
end of the shield volcano building stage and the beginning of the
subsequent mafic and silicic, so-called bimodal,
his
interpretation,
this
expelled during the
shield
onto
volcano,
which
explosive
which
with
Donnelly-Nolan
et
reinterpreted
the
(1983)
age
and
the
occurred.
interpretation
al.
of
Medicine
of
remapped
Mertzman
the
Medicine
distribution
of
formation
of
of
the
a
tuff
-
Km
the
Donnelly-Nolan
as
andesite
and
7
12
was
not
depression
(1977a)
andesite
this
and
andesite
that
responsible
at
tuff.
Lake
tuffaceous unit, suggesting that it is more recent and
eruption
Lake
a depression or caldera into and
extrusion
this
In
unit represented material
collapse
formed
subsequent
concurred
tuffaceous
volcanism.
the
for the
Medicine
Lake.
Anderson interpret the depression or caldera
tectono-volcanic
extensional
basin;
an
interpretation
consistent with the overall extensional regime of the area.
Field Study
Grove, Baker and Donnelly-Nolan carried out field studies of
the
Callahan
flow.
They sampled the area around the one of the
vents, Cinder Butte, through which the Callahan errupted, and the
perimeter
samples.
of
the
flow.
Fig.
2 shows the locations of their
13.
Fig. 2 Sample location map for the 79, 83 and M Callahan series,
previously emplaced lava flows.
pelf =
14.
Analytical Methods
X-ray Fluorescence Analysis
The 79 and 83 series samples were sawn into
slabs
and
saw
marks were removed by grinding on glass plates with 220 SiC grit.
The slabs were wrapped in heavy
crushed
with
cleaned
in
a
plastic
and
linen
sledge hammer on a steel plate.
double-distilled
H20
and
towels
and
All chips were
powdered
in
a
tungsten-carbide shatterbox.
Major-element whole rock
series
were
compositions
on
the
79
and
83
obtained by XRF analysis with the automated Siemens
XRF analyser with on-line data reduction
at
Massachusetts,
Geology
Amherst,
Department
of
the
University
and Geography.
Michael Baker and Daniel Tormey carried out the analyses
Callahan samples.
of
of
the
Splits of the powdered sample were ignited and
mixed with Johnson and Matthey spectroflux 105 in approximately a
1:5
into
ratio,
molds
typically
fused at 1020 degrees C in a Pt crucible, and poured
to
form
displayed
discs
for
deviations
analysis.
Duplicate
of less than 1% for Si02, Ti02,
A1203, Fe203, MgO, CaO, K20, and 2-10% for MnO, Na20,
Standard
9
and
P205.
rock BCR-l was prepared with the same batch of flux and
analyzed simultaneously as a reference standard.
the
glasses
analyses
The average
of
of BCR-l is provided in Table 1, along with the
accepted values of BCR-l (Gladney and Burns, 1983).
The M series
15.
Table 1.
Comparison of U. Mass. analyses of USGS-standard BCR-1
and accepted value of BCR-1 (Gladney and Burns, 1973).
wt. %
oxide
Aanhydrous
accepted
value
U. Mass
avg (9) st dev
Si02
A1203
Fe203
Fe0
MnO
MgO
CaO
Na20
K20
Ti02
P205
54.35
13.63
13.46
0.00
0.18
3.45
6.95
3.27
1.69
2.22
0.37
54.37
13.45
13.30
0.00
0.19
3.38
6.94
3.48
1.69
2.24
0.41
total':
99.57
99.45
.12
.03
.03
.01
.02
.01
.08
.004
.004
.005
16.
samples
(Tables
2
and 5) were analysed at the USGS facility in
Lakewood, Colorado.
Electron Microprobe
Compositions of phenocryst phases in
were
obtained
with
the
MIT
microprobe using on-line data
procedures
the
Callahan
samples
MAC-5
electron
3-spectrometer
reduction
and
matrix
correction
of Bence and Albee (1968) with modifications of Albee
and Ray (1970).
Trace elements
Abundances of Sr were
isotope
dilution
methods
determined
described
in
selected
samples
by
in Hart and Brooks (1977).
Total chemistry blanks during this study for Sr (0.2
ng/g)
were
powders
were
considered negligible.
Isotopes
For Sr isotopic composition analyses,
dissolved
in open beakers with a HF-HC104 mixture and K, Rb, Cs,
Sr, Ba, and
REE
were
separated
on
standard
columns
(Hart
(1980).
Sr isotopic data are normalized to
and
sample
87Sr/87Sr
cation
exchange
and Brooks, 1977), described in detail in Zindler
values
reported
in
Table
86Sr/88Sr
=
0.1194
6 are relative to an
17.
accepted value of .70800 for the Eimer and Amend SrC03
All
Sr
isotopic
composition analyses were performed on a 9" 60
degree radius mass spectrometer (NIMA-B).
87Sr/86Sr
as
standard.
In run
precision
for
represented by 2 sigma mean is typically 0.005% or
better.
Mineral Chemistry and Petrography
Anderson (1941) divided the Modoc
basalts
into
groups based on petrography and flow morphology.
is characterized by pahoehoe
flow
intersertal with few phenocrysts.
surfaces
two
broad
The first group
and
is
dominantly
Lavas that comprise the second
group are largly porphyritic with conspicuous phenocrysts set
a
in
dark aphanitic groundmass of a dominantly hyalopilitic texture
and produce aa to blocky flows.
Anderson (1941) further
that gradations exist between these two types.
claimed
The Callahan flow
was named by Peacock (1931), and described by Anderson (1941)
the
"extensive
flood
near
the northern base of the highland".
The flow is typically aa with local pahoehoe surfaces
within flow compositional variation.
analyses
are
samples with the lowest
smooth,
ropy,
phenocrysts.
were
pahoehoe
presented
Si02
in
Table
content
(e.g.
flows,
reflecting
Samples collected by Grove,
Baker, and Donnelley-Nolan are located in Fig.
chemical
as
with
low
2 and their
2.
In general the
83-18)
flow
The samples highest in Si02 content
bulk
are
from
fronts and few
(e.g.
83-31)
collected from blocky, aa type flows with high flow fronts.
18.
Table 2.
Major element analyses on selected Callahan samples.
----------------------------------------------------------------Oxide
Sample Number:
----------------------------------------------------------------1031M 83-18 79-38b 83-16 947M 83-38 83-35
79-38k 17M
----------------------------------------------------------------Si02
52.10 52.39 53.03 53.24 53.39 53.50 53.68
54.20 54.40
Ti02
1.00 0.97 0.93
0.94
0.94 0.96 0.94
0.93
0.92
A1203
17.90 17.70 17.35 17.44 17.60 17.50 17.43
17.48 17.70
Fe203
9.36 8.86 8.35
8.54 8.67 8.65 8.42
8.42
8.53
MnO
0.15 0.14 0.16
0.15 0.14 0.15 0.13
0.12
0.14
MgO
6.62 6.15 5.70
5.98 5.98 5.97
5.82
5.65
5.67
CaO
9.69 9.24 8.61
8.85 8.98 8.86
8.74
8.67
8.79
Na20
3.44 3.69 3.67
3.53 3.25
3.40 3.63
3.32
3.20
K20
0.62
0.72 0.97
0.88 0.89
0.88 0.94
0.99
0.98
P205
0.18 0.20
0.22
0.19 0.17 0.18 0.17
0.17
0.16
Total: 101.06 100.04 98.99 99.73 99.21 100.05 99.91 99.93 100.49
Oxide
Sample Number:
83-33
686M
83-19
902M
83-34a
718M
83-21
83-23a
Si02
Ti02
A1203
Fe203
Mn0
MgO
CaO
Na20
K20
P205
54.67
0.92
17.40
8.25
0.12
5.56
8.46
3.70
1.06
0.18
54.88
0.92
17.39
8.14
0.13
5.45
8.24
3.44
1.21
0.18
55.12
0.90
17.25
8.01
0.13
5.41
8.16
3.73
1.17
0.18
55.09
0.90
17.40
8.03
0.13
5.43
8.18
3.40
1.22
0.17
55.14
0.92
17.22
8.04
0.13
5.28
8.14
3.65
1.17
0.18
55.30
0.92
17.36
8.03
0.13
5.33
8.04
3.44
1.26
0.18
55.56
0.90
17.28
7.91
0.12
5.30
8.06
3.79
1.24
0.17
55.96
0.89
17.19
7.87
0.13
5.18
7.97
3.67
1.27
0.17
Total:
100.31 99.98
99.86
99.99
100.72 100.29
83-14b
83-15
83-31
974M
57.36
0.87
17.01
7.47
0.12
4.77
7.44
3.74
1.49
0.18
57.16
0.90
16.81
7.36
0.14
4.77
7.29
3.88
1.50
0.18
57.42
0.94
16.77
7.48
0.15
4.47
7.14
3.87
1.53
0.20
57.56
0.90
16.96
7.55
0.12
4.46
7.17
3.57
1.59
0.18
Oxide
100.06
99.95
Sample Number:
83-14a
83-22
83-28
146M
Si02
Ti02
A1203
Fe203
MnO
MgO
Ca0
Na20
K20
P205
56.96
0.86
16.97
7.47
0.14
4.78
7.41
3.77
1.47
0.18
56.92
0.84
16.92
7.43
0.12
4.77
7.45
3.66
1.47
0.16
57.22
0.90
16.93
7.51
0.13
4.66
7.33
3.89
1.47
0.20
57.10
0.85
17.00
7.56
0.12
4.77
7.52
3.32
1.47
0.12
Total:
100.01
99.74
100.26 99.83
100.43 99.99
99.96 100.06
A Total iron reported as Fe304; analyses reported on an
anhydrous basis.
- iPurriO*E~~-'d~"m~urrc~--
~-~-
19.
The samples with intermediate
Si02
content
correspond
to
the
textural gradations between the two types as first pointed out by
Anderson (1941).
Phenocryst Assemblages
All of the samples contained
phenocrysts.
Table
3
between
presents
5
and
10
phenocryst
volume
%
assemblages,
representative electron microprobe analyses are provided in Table
4.
Plagioclase
is the dominant phenocryst phase, and occurs in
all of the samples.
although
its
Olivine
abundance
is
samples.
in
all
the
samples,
decreases with increasing Si02 content.
Augite phenocrysts occur in some
silicic
present
of
Orthopyroxene,
the
intermediate
however,
is
the
pyroxene phenocryst phase in samples with more than 53.5
to
more
dominant
wt.
%
present
in
Si02.
Plagioclase
Two populations of plagioclase phenocrysts
the Callahan flow.
One is characterized by andesine cores (An 35
to 50) with labradorite rims (An 60
bytownite
cores
are
(An
70
to
75)
and
the
other
to 80) and labradorite rims.
by
The fine
laths of plagioclase in the groundmass are typically
labradorite
(An
plagioclase
60
to
70).
Fig.
3
shows
representative
microprobe analyses, projected into the
feldspar
ternary.
The
~~--lr-----
IIX-
20.
Table 3.
Phenocryst assemblages in the
Callahan samples. Samples listed
in order of increasing Si02 wt. %.
Abbreviations: ol-olivine, plagplagioclase, cpx-clinopyroxene,
opx-orthopyroxene.
----------------------------------------Sample
Phenocryst Assemblage
83-18
-------------------, pla
83-16
ol, plag
83-38
ol, plag
83-35
79-38k
83-33
83-19
83-34a
83-21
83-23a
83-22
83-14a
83-28
83-15
83-14b
83-31
83-23b
ol, plag, opx*, cpx**
ol, plag, opxA
ol, plag
ol, plag, opx
ol, plag, opxA
ol, plag, opx
ol, plag, opx
ol, plag, opx, cpxAA
ol0, plag, opx
olA, plag, opx
ol*, plag, opx, cpxA*
ol*, plag, opx
ol***, plag, opx, cpx***
olA**, plag~*, cpx***
A rare, ** extremely rare, AA* present in
basaltic inclusion
21.
Table 4.
Representative analyses of phases from the Callahan Flow
83---------------------18---------------------------------------83-18
--------------------------------------------
Plagioclase
pheno- rim/
cryst groundcore
mass
Si02
A1203
Ti02
Fe0
MgO
CaO
Na20
K20
Mnu
Cr203
Total:
55.97
27.13
52.78
29.53
0.31
0.03
9.12
6.25
0.45
0.67
0.27
12.86
4.31
0.19
99.26
100.59
Olivine
phenocryst
pheno- rim/
core
cryst ground(unzoned)
core
mass
-------------------------------------------------------46.64
39.88 38.95
33.59
0.07
0.25
0.00
0.03
0.54
15.13 18.55
0.19
45.41 41.97
17.15
0.22
0.37
1.95
0.05
0.18
0.31
0.01
0.04
100.10
100.90 100.47
------------------------------------------------
83-19
Plagioclase
Orthopyroxene
-----------------------
phenocryst
core
------------------------
rim
phenocryst
core
rim
groundmass
phenocryst
core
rim
52.77
0.76
0.19
17.04
25.26
2.46
0.02
53.34
0.97
0.24
14.65
26.75
2.45
0.04
53.64
1.06
0.19
13.21
27.99
2.54
0.04
0.52
0.01
0.38
0.14
0.38
0 08
99.04
98.96
99.13
phenocryst
core
---------------------------------------------------
Si02
A1203
Ti02
FeO
Mg0
CaO
Na20
K20
MnO
Cr203
58.29
26.29
55.20
27.46
51.25
29.84
54.12
27.88
50.91
30.05
0.28
0.06
9.15
6.03
0.63
0.86
0.21
11.27
4.86
0.41
0.80
0.23
14.38
3.55
0.10
0.85
0.27
11.80
4.87
0.33
0.58
0.23
13.60
3.81
0.15
Total:
100.72 100.45 100.15 100.12
---------------------------------------------------
99.33
22.
Table 4.
(cont.)
83-19
(cont.)
83-23a
--------------------------------------------------------------Olivine
Placioclase
Orthopyroxene
Si02
A1203
Ti02
Fe0
Mg0
CaO
Na20
K20
MnO
Cr203
Total:
pheno- rim/
pheno- rim/
pheno- rim
cryst groundcryst groundcryst
core
mass
core
mass
core
-------------------------------------------------------------------38.80 37.76
48.16 50.14
54.87 54.82
0.06
0.32
31.85 30.94
1.04
0.98
0.00
0.07
0.16
0.21
16.11
23.23
0.59
0.52
13.43
14.49
44.16 37.66
0.25
0.21
28.07
26.74
0.18
0.31
15.76
14.26
2.23
2.60
2.79
3.61
0.04
0.07
0.08
0.16
0.18
0.34
0.33
0.44
0.00
0.00
0.07
0.05
99.49
99.70
99.85
100.24 100.41
83-23a (cont.)
83-15
Olivine
Plagioclase
Orthopyroxene
Phenorim/
cryst groundcore
mass
Phenocryst
core
rim
57.10
25.92
50.60
29.88
0.33
0.03
9.42
7.28
0.41
0.70
0.19
14.71
3.73
0.17
51.65
1.20
0.16
24.48
20.66
1.34
0.06
53.56
0.89
0.14
19.25
23.79
2.28
0.06
0.77
0.00
0.47
0.00
100.41
100.45
Phenocryst
core
rim/
groundmass
39.59
0.10
0.00
15.18
44.83
0.22
38.99
0.17
0.00
19.16
41.51
0.24
0.24
0.12
0.27
0.05
Total: 100.25
100.39
Si02
A1203
Ti02
FeO
MgO
CaO
Na20
K20
MnO
Cr203
99.48
100.37
99.98
23.
Table 4.
(cont.)
83-15
(cont.)
83-31
Clinopyroxene
Olivine
phenocryst
core
rim
phenocryst
core
rim
phenocryst
core
rim/
groundmass
Si02
A1203
Ti02
FeO
MgO
Ca0
Na20
K20
Mn0
Cr203
50.10
1.71
0.47
11.99
13.86
20.13
0.37
50.76
2.05
0.51
10.58
15.81
18.40
0.29
38.63
0.12
0.01
15.31
44.64
0.23
38.53
0.15
0.03
18.40
41.89
0.26
57.26
26.01
53.66
27.68
50.03
30.83
50.60
30.58
0.39
0.04
8.65
6.41
0.50
1.01
0.19
11.12
4.95
0.38
0.26
0.08
13.76
3.68
0.07
0.24
0.07
13.51
3.63
0.12
0.47
0.00
0.30
0.06
0.23
0.06
0.28
0.05
Total:
99.10
98.60
99.23
99.59
99.27
98.99
98.72
98.75
83-31
(cont.)
Plagioclase
Orthopyroxene
phenocryst
core
rim
Clinopyroxene
phenocryst
core
rim
phenocryst
core
phenocryst
core
rim
Olivine
phenocryst
core
Si02
A1203
Ti02
FeO
MgO
CaO
Na20
K20
Mn0
Cr203
53.78
1.02
0.33
16.41
25.81
2.38
0.06
54.49
2.50
0.28
14.38
25.27
2.72
0.38
52.71
0.72
0.20
22.69
22.37
1.33
0.05
53.18
1.20
0.33
16.15
25.80
2.42
0.06
50.16
3.13
0.87
8.53
15.03
20.56
0.34
37.24
0.09
0.01
26.27
36.56
0.22
0.42
0.04
0.35
0.16
0.62
0.04
0.42
0.08
0.23
0.17
0.36
0.00
Total:
100.25 100.52 100.71
99.65
99.01
100.75
24.
Table 4.
(cont.)
------------------------------------------------------------------
79-38kA
Ground mass phases
Plagioclase
Orthopyroxene
plagioclase
augite
pigeonite
phenocryst
core
rim
Si02
A1203
Ti02
FeO
Mg0
CaO
Na20
K20
MnO
Cr203
51.80
30.20
52.90
1.02
0.27
18.20
22.90
4.57
0.01
0.00
0.38
0.02
57.00
26.90
50.70
31.60
0.84
0.11
13.10
3.66
0.22
51.90
2.58
0.64
10.50
17.60
15.50
0.36
0.00
0.31
0.29
0.32
0.00
9.43
6.16
0.38
0.50
0.09
14.60
2.84
0.12
54.90
0.87
0.20
13.50
28.60
2.22
0.00
0.01
0.27
0.13
Total:
99.90
99.70
100.30
100.20
100.40
100.70
79-38k (cont.)
Olivine
phenocryst
83-23b
Spinel
Plagioclase
Clinopyroxene
Olivine
pheno- rim/
cryst groundcore
mass
Si02
A1203
Ti02
FeO
MgO
CaO
Na20
K20
MnO
Cr203
40.10
0.05
0.00
15.00
45.80
0.23
37.20
0.09
0.05
29.20
33.70
0.24
0.21
1.87
19.60
45.50
1.77
0.16
0.18
0.09
0.40
0.07
0.45
0.25
Total:
101.50 101.00
98.50
50.04
31.13
0.41
0.19
14.87
3.00
0.13
99.76
50.43
3.55
0.59
7.31
16.70
19.96
0.30
38.94
0.06
0.02
19.91
41.27
0.24
0.28
0.98
0.24
0.02
100.10
100.72
A 79-38k analyses taken from Gerlach and Grove (1982)
-.-----I--l-~--LI-arirrV~~gr~~
25.
Ab
Ab
kn
An
Ab
Ab
Or
Fig. 3 Representative plagioclase microprobe analyses projected into the feldspar
ternary. Samples are arranged in order of decreasing silica content from left to
right. An = anorthite, Ab = Albite, Or = orthoclase.
26.
samples
are
circles
represent
analyses,
arranged in order of increasing Si02 content.
and
core
open
analyses,
triangles
plus
signs
Open
represent
rim
represent groundmass plagioclase
compositions.
The plagioclase grains in the andesine group can
3-5
mm
in
looking
phenocrysts
usually
rim.
grain
commonly
reacted
bounderies.
exhibit
out
to
reacted
interiors
to
These
plagioclase
textures,
partially
ranging
reacted
from
zones,
isolated from the groundmass by a more calcic overgrowth
The reacted
opaque
up
size and are most often tabular to equant with wavy,
dissolved
entirely
be
zones
of
intergrowths
Grove,
(Gerlach and
grains
plagioclase
are
filled
pyroxene,
In
1982).
in
with
fine-grained
and
plagioclase
general
the
magnetite
cores
of
this group are normally zoned.
shows a photomicrograph of
a
reacted
plagioclase
nearly
these
Fig.
from
4a
sample
4b is a microprobe traverse of the same plagioclase
83-19.
Fig.
grain.
The compositions of the
reacted
zones
in
the
reacted
grains tend to cluster around one composition within
plagioclase
each sample but vary between different samples.
The second population of plagioclase phenocrysts present
the
Callahan
(An75
to
flow
An85)
compositions
samples
show
which
similar
to
in
is characterized by bytownite cores
normal
zoning
outwards
to
rim
those of the groundmass (An 60 to 70).
In some cases, however, these more Ca-rich grains
are
virtually
27.
Fig. 4 a. Photomicrograph of a reacted plagioclase from sample
83-19. Width of photo = 1.5 mm.
100.
I
I
I
I
75.
rim
eacted
zone
4-)
" se. Be0
core
zone
C
25.Be-
Bel
0.
". e
...
e__ _L
_ _
. 25
_L _
8.58
.
_ _
8.75
i
1. 88
1.25
1. 58
mm
Fig. 4 b. Microprobe traverse of the phenocryst pictured in Fig. 4a. Scale is approximate.
1
1~911~_~
28.
unzoned,
although
calcic plagioclase
plagioclase.
they
are
similar
usually
in
thinly rimmed with a less
composition
to
the
groundmass
This second population of plagioclase grains occur
as coarse laths (up to 2 mm) and
are
containing
brownish black glass.
Although
do
reaction
inclusions
of
dark
plagioclase grains in this group
textures
typically
not
sieve-textured,
exhibit
of the first group, they often have partially dissolved
grain bounderies.
dissolved
Fig.
Ca-rich
5a is a photomicrograph of
plagioclase
grain
from
more
equant
Ca-rich
intergrown with an olivine,
grain
from
is
sample
a
sample
microprobe traverse of the same grain is shown in
second
the
83-23a;
Fig.
pictured
5b.
in
83-18.
traverse of this grain is also provided in Fig.
partially
A
Fig.
a
A
7a,
microprobe
7c.
Olivine
Olivine is present as a phenocryst phase in all the
although
collected,
location.
more
abundance
varies
location
from
to
Generally, the samples with lower Si02 content contain
olivine
olivines are
subhedral
its
samples
to
phenocrysts
up
to
1
euhedral.
mm
than
in
the
more silicic samples.
The
equant
and
maximum
Fig.
microprobe analyses projected into
dimension,
6 shows representative electron
the
pyroxene
quadrilateral.
Core compositions are represented by Xs, rim compositions by open
circles and groundmass compositions by plus signs.
range
The
olivines
in composition from Fo84 in the most mafic samples to Fo72
29.
Fig. 5 a. Photomicrograph of a Ca-rich plagioclase from
sample 83-23a. Width of photo is 1.5 mm.
188I a
---
ore
midzone
n
5L
a.P
*
r
U.
L4
mm
Fig. 5 b. Microprobe traverse of the phenocryst pictured in Fig. 5a. Scale is approximate.
30.
Wo
Wo
En
En
Wo
Fs
Fig. 6 Representative olivine and pyroxene microprobe analyses projected into the
Wo-En-Fs ternary. Samples are arranged in order of decreasing silica content from
left to right. Wo-= wollastonite, En = enstatite, Fs = ferrosilite.
31.
in the most silicic samples, and are commonly unzoned except
a
thin
Fe-rich
composition.
for
rim which corresponds to the groundmass olivine
In most cases
the
olivine
phenocrysts
are
more
7a is a photomicrograph of an olivine phenocryst
from
magnesian rich than the olivines in the groundmass.
Fig.
one
of
the
most mafic samples 83-18;
the same grain is shown in Fig.
this
7b.
a microprobe traverse of
The euhedral morphology
of
olivine is typical of olivine phenocrysts in the more mafic
samples.
Embayed textures and corroded edges are typical of
the
olivine phenocrysts in the more silicic samples.
Olivine and Ca-rich plagioclase phenocrysts often
small
crystal aggregates in the Callahan flow.
consist of sieve textured lathlike plagioclases
olivine
(Fig.
8),
occur
These aggregates
intergrown
and
less
commonly clinopyroxene.
cases the aggregates look
like
small
plagioclase
and
spherical,
intersertal
to
plagioclase,
basalt-like
coarse
grained
approximately
intergranular
olivine,
basaltic
Table
Figs.
3,
and
3 and 6.
In some
clusters
10
cm
texture,
clinopyroxene
and
blob.
of
their
83-23b
is
in diameter, exhibits
and
light
Representative microprobe analyses of these phases
in
with
olivine, plus or minus clinopyroxene and glass.
Sample 83-23b is a
roughly
as
consists
brown
are
of
glass.
provided
compositional ranges are summarized in
32.
Fig. 7 a. Photomicrograph of Mg-rich olivine and Ca-rich
plagioclase from sample 83-18. Width of photo is 1.5 mm.
_
core
'
-E
-
midrim
-I,
C
C s. Bt
0
0
0
S.I M1
mmMMl
U.
U
,
Ig.
58
1. 55
mm
Fig. 7 b. Microprobe traverse of olivine pictured in
Fig. 7a. Fo = forsterite. Scale is approximate.
~~D
~
~acr*l~s.~~33.
C
pictured in Fig. 7a.
Scale is approximate.
mm
Fig. 7 c. Microprobe traverse of Ca-rich plagioclase
pictured in Fig. 7a. Scale is approximate.
34.
Fig. 8 Photomicrograph of olivine and plagioclase from
sample 83-19. Width of photo is 1.5 mm.
I.------r-irr~--r-----xr- -~
lyl~-~-rralrri
x~-rssl
-~nr~ls~Y~;-r-
35.
Clinopyroxene
Clinopyroxene occurs infrequently as a phenocryst
the
Callahan flow.
subhedral
reacted,
in
the
basaltic
dissolved
basaltic inclusion present
discreet,
in
It is present as medium grained, anhedral to
crystals
partially
phase
subhedral
in
inclusion
finer
grains
83-31
and
is
(83-23b),
as
in the microscopic
also
present
as
to euhedral phenocrysts in some of the more
silicic samples (e.g.
83-15).
The clinopyroxene phenocrysts are
always slightly corroded in appearance and tend to be enriched in
iron
relative
inclusions.
projected
to
the
Electron
into
compositions
the
are
clinopyroxenes
found
in
the
basaltic
microprobe analyses of clinopyroxenes are
Wo-En-Fs
ternary
represented
as
in
open
Fig.
6.
triangles
Core
and
rim
compositions as open boxes.
Orthopyroxene
Orthopyroxene occurs as a phenocryst phase in
Si02 content greater than approximately 53.5%.
phenocrysts range in appearance
coarse
lath-like
grains.
from
stubby
Compositions
samples
with
The orthopyroxene
equant
of
grains
to
orthopyroxene
phenocrysts range from En 76 to En 52;
in a single
En72
analyses of orthopyroxene
to
En59.
Electron
microprobe
sample
phenocrysts are projected into the Wo-En-Fs ternary in
The symbols are similar to those used for clinopyroxene.
Fig.
from
6.
Samples
36.
83-31 and 83-19 contain more than one population of orthopyroxene
phenocrysts;
in
83-19
the
two
populations
compositionally simmilar orthopyroxene rims.
two
populations
are
mantled
In the samples with
of orthopyroxene phenocrysts, the orthopyroxene
rim composition is more Fe rich than one population and
rich
then
by
the other.
more
Mg
In general, the orthopyroxene phenocrysts
appear to be partially dissolved
and
corroded,
the
degree
of
dissolution and corrosion increasing with decreasing bulk silicia
content.
Figs.
The orthopyroxenes from samples
9a
and
9b
changing Si02 wt.
orthopyroxene
83-23a
and
83-31
in
exhibit this correlation between texture and
%.
83-23a has
phenocrysts
83-31 has 57.42 wt.
55.96
wt.
%
Si02
and
the
are partially dissolved and corroded.
% Si02 and the orthopyroxene phenocrysts are
coarser and less reacted in appearance.
The abundance of orthopyroxene
decreasing
silica
content.
phenocrysts
samples
with
Si02 wt.
only
thin section.
in
the
an
with
abundant
% greater than 56;
in
% less than 56 orthopyroxene phenocrysts
become quite rare until, at an
83-35)
is
Orthopyroxene
phenocryst phase in samples with Si02 wt.
decreases
Si02
wt.
%
of
53.68
(sample
one small dissolved orthopyroxene is present in the
The bulk of the orthopyroxene phenocrysts
present
Callahan flow have chemically homogenous cores, and both
Fe rich and Mg rich rims, depending on the sample.
phenocrysts
occur
as
discreet
grains,
basalt-like aggregates described above.
never
Orthopyroxene
present
in the
37.
Fig. 9 a. Photomicrograph of orthopyroxene phenocryst from
sample 83-31. Width of photo is 1.5 mm.
Fig. 9 b. Photomicrograph of orthopyroxene phenocryst from
sample 83-23a. Width of photo is 1.5 mm.
__p~~_1~1~
38.
Opaques
Opaques occur ubiquitously in the groundmass but rarely,
ever,
as
a
phenocryst phase in the Callahan flow.
silicic samples (Si02 wt.
titaniferous
magnetite
if
In the more
% > 57), discreet, small, octahedra of
are
discernable
from
(Gerlach and Grove, 1982, Mertzman, 1977).
In
the
the
groundmass
more
mafic
samples the magnetite is indiscernable from the groundmass.
Groundmass
The groundmass of the Callahan flow consists primarily of
glassy
to
microcrystalline, intersertal to intergranular matrix
consisting of abundant lath-shaped plagioclase and
to
quenched
part,
most
titaniferous
of
small
equant
textured olivines set in an intergrowth of, for the
The groundmass pyroxenes have
magnetite and glass.
intergrown
augite
clinopyroxene,
intergrown
indistinguishably
been analysed in sample 79-38k
consist
a
(Gerlach
and
and
Grove,
pigeonite.
1983)
and
Compositions of
groundmass plagioclase and olivine are plotted in Figs.
3 and 6.
Silicic Inclusions
Silicic inclusions were collected from the Callahan flow
Cinder
Butte
(83-37
and
902Mb,
small (5-10 cm) blocks of silicic
Fig.
scoria,
2).
at
The inclusions are
fused
rhyolite,
and
^_~'ll~-~--~P11-
----LI--ili-
~~"Y
39.
intermingled
The major element
silicic and more mafic material.
addition
chemistry of 902Mb is presented in Table 5, in
of
analyses
silicic
types.
inclusions
silicic
(An
18-20)
flows.
in the Callahan flow are of two general
by
One type (83-37a) is characterized
phenocrysts
the
several older rhyolite and dacite flows from around
the Callahan flow, and an average of those older
The
to
in
buff
colored,
rare
plagioclase
translucent
highly
vesicular glass and the other (83-37k) by plagioclase phenocrysts
(An
18-22)
partially dissolved lithic fragments in densely
and
The lithic fragments consist predominantly
welded tan glass.
alkali
feldspar
of
are altered to light rust in
and
silica
with
color.
The phenocryst assemblage in the mafic
scoria
(83-371)
portions
of
the
of plagioclase (An 37-87) and olivine
consists
(Fo 75-83).
Major Element Chemistry
in
Variation diagrams for the major elements
flow
are
MgO
vs.
in
variation in composition
demonstrated
linearly
to
the
decreasing
with
Mg0.
are
trends
by MgO variation diagrams.
decreasing MgO;
MgO.
and
Callahan
The lavas show a significant
10.
Fig.
the
Si02
especially
well
The major elements vary
and
K20
increase
with
and FeO, Ca0, and A1203 decrease with decreasing
The behavior of Na20 is more variable, however it does tend
increase
with
decreasing
decrease with decreasing MgO.
MgO.
Ti02
shows
a very slight
IIII-~C^I*I1^I--~YII--^
i-mi 1~1~-~ 11111~-111i~-_IU^1_
40.
Table 5. Major element analyses of silicic material in and
around the Callahan flow.
----Oxide
Sample Num----ber:--------------
Oxide
Sample Number:
----------------------------------------------------------(1)
(2)
(3)
(4)
(5)
(6)
(7)
avg of
902Ma 18M
19M
142M
684M
685M
830M
(2)-(7)
Si02
74.3
72.6
72.1
75.4
72.4
74.9
73.6
73.5
Ti02
0.13
0.43
0.42
0.19
0.30
0.19
0.29
0.30
Al203
13.2
14.4
14.2
13.4
13.8
13.3
14.3
13.9
Fe203
1.70
2.39
2.36
1.33
3.07
2.12
1.74
2.17
MnO
0.02
0.04
0.04
0.02
0.03
0.02
0.18
0.06
MgO
0.39 0.39
0.57
0.22
0.39
0.24
0.28
0.32
CaO
1.27
1.22 1.21
0.82
1.14
0.81
1.09
1.05
Na20
4.03
4.40 4.39 3.71
4.11
3.81
4.14
4.09
K20
4.66
4.32
4.33 4.62
4.62
4.68
4.22
4.46
P205
0.05
0.06 0.06
0.02
0.07
0.00
0.12
0.07
-----------------------'---------------------------
Total:
99.9
100.3
99.5
99.7
99.9
100.1
100.
99.9
---------------------------------------------------------
*Total iron reported as Fe304; analyses reported on an anhydrous
basis.
41.
2.0
s.u
2.0
1.5
1.5
6.0
a
1.0
M
X
A,
1.0
?.
0.5
0.5
6.0
0.0
4.0
59.0
7.0
6.0
5.0
4.0
16.0
58.0
0.0
8.18
9
57.0
17.5
'
A
56.0
00 n
55.0
17.0
II
DII
x54.0
52.0
51.0 7.o
16.5 I
AL1
A
A
53.0
LI
,
1.0
5.0
4
10.0
16.0
-
7.0
6.0
5.0
4.0
.1
A
A
4.0
U.
*
9.0
M
A
,SIS-A
w
A
A
A
UI
UI
.6
L 15
6.0
7.0 '
7.0
I
6.0
I
wt. X MgO
5.0
4.0
"'
L141
7.0
5.0
wt.
5.0
4.0
7.6N
w.
M
5.A
X MgO
wt.
Fig. 10 Major element oxides plotted against wt. % MgO.
triangles represent Callahan samples.
Z M9 0
Open circles and
4.80
42.
The scatter in the Na20, MnO,and
from
large
analytical
P205
uncertainties.
versus
MgO
The accuracy of the XRF
analytical technique for these elements is only within
the
amount
present.
An
between-laboratory error.
analysed
Colorado,
for
major
and
a
1
Eight of the twenty four samples
elements
systematic
gives
2-10%
of
additional problem with Na20 involves
were
at the USGS facility in Lakewood,
difference
analyses and those obtained at U.
Table
results
accepted
exists
between
these
Mass., Amherst.
values
for
the
BCR-1
standard
(Galdney and Burns, 1983), and an average of BCR-1 analyses taken
along
with
University
the
analyses
collected
by
of Massachusetts at Amherst.
Baker
and
Tormey
at
The samples analysed at
University of Massachusetts are consistently high with respect to
the
accepted
values
for
BCR-1.
The samples done at the USGS
facility in Lakewood, Colorado are consistently low with
to
Na20
(approximately
5
%,
Donnelly-Nolan, pers.
commun.).
After normalizing the Na20 values to the accepted value
for
BCR-1
the
data
are
respect
of
Na20
better constrained to a linear trend.
This correction was carried out before the data set was modeled.
Trace Element and Isotope Study
Introduction
Medicine Lake
volcano
has
been
the
subject
of
several
43.
earlier
geochemical studies which incorporated trace-element and
isotopic
analyses
(Barsky
Mertzman
1977a,b,
1979,
1975,
Condie
Metrzman,
and
1975,
Hayslip,
Grove et al., 1982).
1981,
a
Trace element information on the Callahan flow is incomplete;
compilation
of
trace
element
data collected in this study and
taken from the literature is presented in Table 6.
will
focus
on
relative
roles
dominate
in
the
which
elements
of
the
are
useful
differentiation
formation
of
the
section
in identifying the
processes
flow.
This
The
preliminary study of 87Sr/86Sr isotopic ratios are
to
thought
results
of a
presented
in
Table 7, and will also be discussed.
Trace Element Chemistry
Element-MgO diagrams are presented in Fig.
Zr
behave
(i.e.,
incompatibly
differentiation proceeds).
(i.e.,
are
enriched
Rb, Ba
in the liquid as
V and Ni exhibit compatible
behavior
(REE)
show
slight
enrichment
as
of
MgO
Y and Sr are scattered on this type of diagram.
A chondrite normalized plot of the rare earth elements of
subset
and
depleted in the liquid as differentiation proceeds).
The rare earth elements
decreases.
are
11.
the
Callahan
samples is presented in Fig.
samples are moderately enriched in the light rare earth
relative to the heavy rare earth elements.
12.
a
The
elements
44.
Table 6. Trace Element Data for Callahan Samples
----------------------------------------------------------------------------------------------------------------------------------Sample
902Ma
146
686M
17M
718M
902Mb 146M CallahanA
----------------------------------------------------------------------------------------------------------------------------------Si02
74.5
54.00 54.30 54.40 54.80 55.20 57.10 53.50
Rb
130
28.2
33.3
27.9
36.6
35
41
33
Sr
46
315
338
292
359
322
310
356
Ni
Zr
Ba
La
Nd
Sm
Eu
Yb
225
625
35.1
29
6.45
0.34
4.59
293
9.6
14.0
3.6
1.09
2.54
Sample
SM-75
SM-75
-29A~* -34AA
305
10.6
13.9
3.5
1.09
2.6
SM-75
-35A-A
251
8.7
11.3
2.9
0.98
2.2
SM-75
-61A
340
11.2
14.4
3.7
1.13
2.64
SM-75
-63AA
316
11.1
15.0
3.5
1.0
2.44
340
11.0
11.5
3.0
0.98
2.26
SM-75
SM-75
-137AA -213AA
61
166
246
9.2
3.9
1.1
2.5
ML-74
-67AA
Si02
54.70
54.20
53.90
57.00
56.40 57.40
51.10
52.80
Rb
36
29
34
48
45
46
15
23
Sr
Ni
V
Zr
Y
Ba
372
51
140
152
5
327
375
52
137
146
22
290
373
52
128
138
20
305
350
54
140
188
18
380
348
51
123
164
12
377
350
50
140
203
25
424
399
62
174
153
16
208
388
62
152
145
9
279
Sample
83-18
83-16
83-14a
83-31
Si02
Sr
52.39
316
53.24
349
56.96
319
57.42
320
A Condie and Hayslip
AA Mertzman (1977)
(1975)
83-23b
375
83-37a
49.8
45.
Table 7.
87Sr/86Sr ratios for selected Callahan Samples
Sample:
83-23b
83-18
87Sr/86Sr
.70368 .70371 .70367 .70372
* analytical error is +or-
83-16
83-14a
83-31
83-37a
.70373 .70385
3 in the 5th place.
Fig. 11 Trace elements (ppm) plotted against wt. % MgO. Boxes = U.S.G.S. data, triangles = Mertzman
(1977) data, circles = Condie et al. (1976) data, diamonds = data from this study.
fRS
Ui
26m
liuu
U
198.
A
A
188.
£.
178.
E 160.
55.
28. eA
148.
A
A
zF1
A
.5
A
138.
A
58. 2-
z
A
A
A
120.
ff
B
IA
I
I
I
7. B
6.5
6.0
•
118.
45.
I
7.5
I
I
7.8
6.5
6. 8
I
I
I
I
5.5
I
S. 8
4. 5
4
3100
'.5
5.5
5.0
4.5
7.5
400.
LF.
-.
,,J
I
400
0
7. 8
6.5
6.8
5.5
I
i
I
5. 8
I
4.5
I
4. 8
I
A
375.
280. E
358. e
E
E
C.350.
N.*
El
175.
288. EB
0-
3908.
AAI
200a.
"C0
B
I
I
7.5
7. 8
6.5
Lt.
I
6.
1
01
A
158. L
El
300.
325.
It.
El
I
5.5
% MgO
I
5. 8
I
4.5
4
7
rI
A
A88
7. B
6.5
wt.
6.
5.5
% MgO
5.0
4.5
4.0
7.
7.0
6.5
wt.
6.
5. 5
X MgO
5.
4.5
4.
Fig. 11
(continued).
15.
II.5E
I
I
p
I
I
a
I
m
0]
11. BE
14.
0
E
E lo.
CL
v
Ie.
10.
0L
0L
13. 0-
E
3. s-
0
12.
0-0
U
0
E
-I
z
U)
3. el
9.
0I
Ia
g
5. 5
5. 8
.
.
9.
a.
0
1
7. 59
7.88 g
.58
8.88
I
I
5.5
5.a
4.
!
4. 18
9.
S
7. 5
7.
I
I1I I
I
6. 5
4.5
5. 8
5.5
6.
-
4.
8
39.
I
1.
I
I
I
7.5
7.
6.5
0
.
.
6.
.
4.5
4,
I
m
Aie
25.
m
A
28.
1.
Ar
rA
15.E
0
1. 9
Ar
rA
18.
0
m
I I
8.
7.50
7. 08
6.58
wt.
8.88
5. 5
% MgO
5.0
4. W
4.
,,
iu
2. 'II I
7. 5
7.0
6.5
wt.
i
6.
A
5.
1
I
5. 5
% M0
I0
5.0
4.5
4.8
"I
8. 01
7.5
7.
8
6.5
wt.
6.8
5.5
% MgO
5. 8
4.5
4. 8
:
48.
Ca llahan
50.
20.
10.
5.
2.I
Ir
LaCePrNcFmSrrEuGdTbD>HoErTmYbLu
Fig. 12 Chondrite normalized rare earth element curves
for selected Callahan samples. Normalization after Frey
et al. (1968).
0. 70400
col lahan
0. 70990L
0.
70 3 8
A
0. 70370L
A,
A
0. 70360
0. 7r03501
1I.01001
1
I
200. 000
ppm Sr
Fig. 13. Sr (87/86) isotopic variation vs. ppm Sr.
400. 000
49.
Strontium isotope variation
Fig.
the
13 shows a plot of 87Sr/86Sr ratios versus ppm Sr
Callahan
flow.
The
variation
of
for
87Sr/86Sr ratio is not
significantly greater than analytical error.
*_~L~YI _~_L__
50.
Discussion
Four general petrogenetic models for andesite
presented
in
the
introduction:
primary
basaltic
material,
were
1) partial melting of hydrated
mantle or subducted oceanic crust, 2) fractional
of
genesis
3)
crystallization
assimilation
of
a crustal
component by parental basaltic magma, and 4) mixing of basalt and
rhyolite.
of
the
In the following section the petrogenetic implications
four
end-member
models
are
discussed
and
their
applicability to the genesis of the Callahan flow evaluated.
Partial Melting of
Hydrated
Mantle
or
Subducted
Oceanic
Crust
This model proposes that compositional variations
generated
in
subduction
alumina
volcano.
in
lavas
regimes result from varying degrees of
partial melting of hydrated mantle or
High
in
subducted
oceanic
crust.
basalt (HAB) is ubiquitous at Medicine Lake shield
Basaltic andesite, andesite, dacite and rhyolite
occur
proportions roughly consistent with their derivation from the
HAB by varying degrees of fractional
al.,
1982).
The
crystallization
(Grove
et
composition of the HAB is consistent with its
derivation as a partial melt of some package of subducted oceanic
plate
and
hydrated
mantle, and HAB is postulated as the parent
magma in the general fractional crystallization
for
the
evolution of the volcano.
scheme
proposed
Because of its evolved major
51.
element chemical
Callahan
flow
relative
composition
is
interpreted
to
the
HAB
the
lava,
to be related to the HAB partial
melt by crustal level processes.
Fractional Crystallization of Primary Basaltic Material
Grove
controls
and
on
systems.
Baker
discuss
the
phase
equilibrium
differentiation in the tholeiitic and calc-alkaline
The
following
crystallization
the
(1984)
Callahan
of
consideration
of
fractional
a mantle-derived basaltic parent to generate
flow's
chemical
variations
parallels
their
discussion.
One-Atmosphere Phase Relations in the Calc-alkaline System
The Modoc intersertal-subophitic high alumina basalt
HAB)
is
chosen
as representative of the parental magma type of
the Callahan flow because it was emplaced in the
the
Callahan
and
In
same
cycle
as
because HAB of similar bulk compositions have
been erupted throughout the
1982).
(Modoc
volcano's
history
(Grove
et
al.,
general, the pre-shield HAB units (Anderson's Warner
basalt) and later Modoc HABs are high in MgO (8.5 to 10 wt.%) and
A1203
(17
to
21
wt.%)
with
Si02
contents of 47 to 49 wt.%,
Fe0+Fe203 of 8 to 11 wt.% and low K20 (0.10 to 0.20 wt.%).
Grove
et
al.
(1982)
determined
the
one-atmosphere
52.
equilibrium
of
crystallization
oliv+plag
crystallization
crystallization
(see Fig.
liquid
Parental
olivine
sequence for HAB, to be an interval
along
the
followed
aug-oliv-plag
by
cotectic.
14), olivine+liquid react to form
becomes
enriched
compositions
and
in
iron
which
pigeonite
and
at
project
on
three
At point A
This
path
compositions
is
lie
which
below
the
a constant silica value.
above
the
join
this projection (i.e.
sketched
and
pigeonite
alumina basalt composition) will crystallize completely
point.
phase
in
Fig.
14.
this
join
will
between
the high
at
this
Qtz normative
continue
to
crystallize at point A until all the olivine has reacted with the
liquid,
at
which
aug-pig-plag
point
the
liquid
will
down
proceed
the
cotectic, crystallizing the three phases, until the
liquid is exhausted.
Under conditions of low pressure fractional
an
HAB
crystallization
liquid is not constrained to remain at reaction point A.
If the crystals separate perfectly then the
aug-pig-plag
cotectic,
liquid
follows
the
percipitating the three phases, until it
is exhausted.
Effects of Elevated Pressure on Phase Relations
The effect of increased pressure under anhydrous
in
the
conditions
system Fo-Di-Si02 (Kushiro, 1969) and in natural basalts
(Kushiro, 1974;
Bender et al.
1978;
Stolper, 1980;
Takahashi
53.
Cpx
aug
A
HAB
pig
B
oliv
opx
Oliv
Qtz
Fig. 14 1 atm. plagioclase-saturated boundaries in the oliv-cpx-qtz pseudoternary. The projection scheme employed was based on oxygen units (Grove et
al. (1982). Point A is a reaction point where oliv+liq = pig+aug+plag and B
is also a reaction point where oliv+liq = opx+pig+plag. HAB represents the
field of High Alumina Basalts postulated as the parental liquids at Medicine
Lake. Heavy arrows indicate an equilibrium crystallization path for a HAB
parent. See text for discussion.
54.
and
Kushiro, 1983) is to shrink the olivine primary phase volume
and expand the orthopyroxene
phenomenon
is
and
(Kushiro,
discussion of the
under
crustal
at
1972).
The
fractional
conditions
the
one
set
of
expense
volumes.
of
conditions
crystallization
This
the
the
pyroxene
applicable
to
of
magma
basalt
a
would be elevated pressure (1-5 kbar)
and water undersaturation (Grove
shows
phase
reversed under conditions of H20 saturation:
olivine phase volume expands
volumes
augite
and
hypothetical
Baker,
cotectics
1984).
Fig.
15
and reaction curves
proposed by Grove and Baker (1984) to exist under such conditions
based
on
the
5
(1983) and
the
Rutherford
(1983).
kbar
water
anhydrous
data of Takahashi and Kushiro
undersaturated
results
of
Spulber
and
Major element compositions for the Callahan
flow samples and the field of
postulated
parental
compositions
similar to the Modoc HAB found at the volcano are also plotted on
this diagram.
units
and
The projection scheme employed is based on
involves
the
reduction from 10 component space to 4
component space and then further projection through
components
(in
this
case
one
of
the
plagioclase) onto 3 component space.
Although this projection clearly prevents rigorous
the
oxygen
treatment
of
total chemical variation of the flow it provides a framework
within which the variation can be visualized.
Although detailed information on the
followed
by
undersaturated
natural
basalt
conditions
is
at
not
crystallization
elevated
pressures
available
some
paths
and
H20
general
55.
Cpx
aug
high
pressure
cotectics
callahan
oliv
Oliv
*
B
opx
Qtz
Fig. 15, Same projection as Fig. 14 with high pressure cotectics sketched in
(after Grove and Baker, 1984). Point A' is the high pressure reaction point
where oliv+liq = aug+pig+plag. The HAB field is shown, in addition to the
Callahan data array. Heavy arrows indicate a high pressure fractional crystallization path for a HAB parent. See text for discussion.
56.
characteristics
of higher pressure crystallization processes can
be inferred.
During equilibrium crystallization, a HAB
the
Fig.
one
in
as
15 will initially crystallize olivine (+ or -
plagioclase), moving the liquid
towards
such
composition
the oliv-aug-plag cotectic.
away
from
olivine
When the liquid reaches the
cotectic augite joins the crystallizing assemblage.
The
liquid
proceeds along the cotectic, bypassing the reaction point present
at
lower
pressures,
until
it
reaches
the
oliv-aug-plag-pig
reaction point, and pigeonite joins the crystallizing assemblage,
through the reaction olivine+liquid going to pigeonite.
case
of
the
basaltic
remains at this
point,
parent
plotted
reacting
with
in Fig.
olivine
In
the
15, the liquid
to
crystallize
pig+aug+plag, until it is exhausted.
Fractional crystallization of the basaltic parent plotted in
Fig.
will
15 at elevated pressures and H20 undersaturated conditions
proceed
discussed
similarly
above,
until
to
the
the
equilibrium
crystallization
reaction point is encountered.
this point, if fractional crystallization is perfect, the
will
At
liquid
not be constrained to remain at the reaction point but will
proceed down the aug-pig-plag cotectic towards the quartz apex of
the ternary diagram.
As is evident from the data plotted
Callahan
flow,
liquids
fractionating
cotectic at higher pressures
are
on
Fig.
15
for
the
along the oliv-aug- plag
capable
of
moving
into
the
57.
basaltic
andesite field, at which point they separate from their
oliv-aug- plag residue and move to
a
shallow
magma
reservoir.
The basaltic andesite liquids may errupt to the surface or evolve
further by low-pressure fractionation.
polybaric
fractional
production of
some
In
this
manner,
then,
crystallization may be responsible for the
calc-alkaline
series
lavas
from
basaltic
parent magmas (Grove and Baker, 1984).
Of specific interest here, however, is whether
or
not
the
Callahan flow represents some portion of the path followed by the
liquid during fractional crystallization of a HAB parent
combination
in
the
of
pressures.
Callahan
crystallization
flow
was
process
at
any
If the range of compositions present
generated
by
some
fractional
from a basaltic liquid then the samples
should fall along
the
phases,
case postulated to be an interval of oliv+plag
in
this
path
constrained
by
the
fractionating
fractionation followed by cotectic precipitation of oliv+plag+aug
14
(Figs.
and
15).
During
the
initial phase of fractional
crystallization at any of the pressures discussed
liquid
evolves
away
from
the
thus
far
the
HAB composition on a trajectory
directly away from olivine towards the cpx apex of
the
ternary.
This trajectory differs significantly from the trajectory towards
the qtz apex which characterizes the Callahan
reaching
the
ol-aug-plag
cotectic
Only
upon
will the liquid evolve in a
direction compatible to the evolutionary trend
data.
data.
of
the
Callahan
If the Callahan flow samples are related one to the other
58.
through
fractional
oliv-aug-plag
crystallization
cotectic,
however,
along
one
a
higher
would expect to see some
evidence of augite in the phenocryst assemblage.
assemblage
in
plagioclase
and
phenocryst
phase.
the
Callahan
olivine.
consists
Clinopyroxene
It
phenocrysts in samples
flow
occurs
of
as
pressure
Si02
phenocryst
predominantly
is
rare,
intermediate
The
not
of
a
common
discreet,
rounded
content,
in
the
basaltic inclusion 83-23b and in the microscopic basaltic bleb in
83-31.
For
any
of
the
clinopyroxene
phenocrysts
to
be
representative of clinopyroxene fractionation, however, they must
be of appropriate composition to have been
the
fractionating
compositions
Callahan
of
liquid.
Fe-Mg
samples
Mg
equilibrium
in
presented
equilibrium
with
selected
in Table 8, in addition to the
observed phenocryst compositional ranges for each sample.
numbers
were
calculated
assuming
that
represent liquids, and the equilibrium
predicted
according
to
the
augite
and
orthopyroxene.
et
phase
al.
The Mg
the bulk rock analyses
experimental
coefficients determined by Grove
with
s (Mg/Mg+Fe) and the predicted
phases
are
in
compositions
exchange
(1982)
for
were
partition
olivine,
Over the compositional range of the
flow, compositions of olivine, clinopyroxene and orthopyroxene in
equilibrium
significantly.
82
to
with
the
bulk
rock
compositions
do
not
vary
The forsterite content in the olivine varies from
80%, the Mg/Mg+Fe ratio in clinopyroxene from .86 to .84,
59.
Table 8.
Mg #sA of Callahan samples, their predicted
equilibrium Fe-Mg phases*A and the observed compositional ranges of Fe-Mg phases in selected samples.
Sample #:
83-18
83-19
83-23a
83-15
83-31
Si02
52.39
55.12
55.96
57.16
57.42
Mg #: 0.579
0.572
0.570
0.562
83-23b
0.542
Mg/Mg+Fe values for predicted equilibrium phasesAA
ol
cpx
opx
0.82
0.86
0.83
0.82
0.85
0.83
0.82
0.85
0.83
0.81
0.85
0.83
0.80
0.84
0.81
Mg/Mg+Fe values for observed phenocryst phases
ol
cpx
opx
.76-.84
.74-.84
.72
.71-.79
.79-.84
.78-.79
.80-.84
.67-.71
.53-.68
.70-.71
.76
.61-.75
.77-.80
.79-.82
* Mg # is calculated as Mg/Mg+Fe
AA Mg # for predicted equilibrium phases are calculated
using the exchange partition coefficients of Gerlach and
Grove (1982): ol-0.29, cpx-0.23, opx-0.27.
60.
and the same
observed
ratio
from
.83
to
.81
in
orthopyroxene.
Mg numbers for clinopyroxenes in the flow, ranging from
.67 to .82, are not appropriate for clinopyroxenes
from
liquids
similar
to
the
Callahan
of
equilibrium
compositions
of
The olivines
83-31,
predicted.
predicted as equilibrium compositions.
span
The
orthopyroxene compositions are deficient in Mg in
those
fractionating
samples.
present in each sample, with the exception
range
The
the
observed
comparison
to
The results of the
Mg number calculation suggest that of the Fe-Mg phases present in
the
Callahan
flow
only
olivine
is
appropriate
as
having
fractionated from Callahan liquids.
A similar analysis of the plagioclase
in
the
Callahan
flow
compositions
suggests that plagioclase phenocrysts in
equilibrium with a liquid similar to 83-18
depending
on
the
were
2.3
are
An-78
the
exchange
partition
and 3.25 respectively).
the
more
mafic
samples
(83-18,
plagioclase, then, is appropriate as
phase.
The
An-85
equilibrium
plagioclase
coefficients
As is obvious from Fig.
3, plagioclase phenocrysts in this composition range
in
to
KDNa-Ca (as defined by Grove and Baker, 1984)
used (in this calculation
used
present
a
83-23a).
potential
for
a
are
common
Anorthitic
fractionating
liquid similar to
83-31, assuming a Ca-Na exchange partition coefficient of 2.3
An-72, more An rich than those observed (An-67) in 83-31.
If the
change in composition that accompanies fractionation changes
KDNa-Ca,
then
plagioclase
compositions
is
the
similar to the Ca rich
61.
phenocrysts observed in
having
the
83-31
are
consistent
with
crystallized from a liquid similar to 83-31.
their
Plagioclase
phenocrysts in the range of An-30 to An-60 occur in every
sample
in the Callahan flow and could not have fractionated from liquids
similar to those same samples.
composition
range
are
not
Plagioclase phenocrysts
Ca
rich
enough
to
in
have
this
been in
equilibrium with the range of liquids represented in the Callahan
flow.
Fractional Crystallization Model
Attempts were
present
in
the
fractional
made
to
Callahan
fit
flow
crystallization.
the
compositional
variations
by a model designed to simulate
The
objective
of
the
modeling
process was to determine if fractional crystallization of a phase
assemblage consistent with the phases present in the
generate
the
major
element
weight units, and was
solids
one
from
of
the
an
most
carried
variations.
out
in
flow
could
The calculation uses
increments
by
removing
initial composition modeled after sample 83-18,
mafic
samples
in
the
Callahan
data
set.
Equilibrium phase compositions were removed at each fractionation
increment
using
partition
coefficients
experimentally
partition coefficient for
determined
for olivine of 0.29.
plagioclase
was
Fe-Mg
exchange
The Ca-Na exchange
set
at
3.25.
The
calculation was carried out in increments of 5 wt.% solid removed
from the residual liquid.
The liquid composition was
determined
62.
by
multiplying
the
wt.%
oxide
in
proportion of the phase times 0.05.
each
phase
The resultant
times
weight
liquids
were
normalized to 100%.
Based on the fractionation path discussed above for
similar
to
liquids
83-18, and on the phenocryst proportions observed in
the flow, the modeled fractionating assemblage was calculated
be
75
% plagioclase and 25 % olivine.
the trends of major element oxide vs.
phases
having
joined
the
evolution of the Callahan
removed
An-83.5;
from
the
The continuous nature of
Mg0 argues against any new
fractionating
flow
assemblage during the
liquids.
The
initial
the olivine composition removed at the
last
provides the results of this model in the form of
oxide verses MgO variation diagrams.
increment
A1203
enriched
enough
major
16
element
observed
in
the
and MgO are depleted too rapidly, CaO is
not depleted rapidly enough, Na20 enriches too
not
Fig.
The trends generated by the
fractionation model do not resemble the trends
flow.
olivine
liquid was Fo-81, the initial plagioclase was
was Fo-65, the final plagioclase composition was An-75.
Callahan
to
rapidly,
K20
is
and Ti02 and FeO both become enriched with
decreasing MgO, instead of being
depleted
as
in
the
Callahan
flow.
The failure of the fractionation model
expected
in
light
crystallization
path
of
the
discussion
followed
by
a
employed
is
to
be
above of the fractional
liquid
of
an
initial
~_,..----....;~_i_;__....~
,_____._..__
. --
-.-.11
~_
63.
2.0
10.0
1.5
9.0
Ii
I
1.0
a.0-
0.5
II
0.0
7.0
5.0
6.0
59.0
4.0
3
a"
58.0
57.0
56.0
<' 15.0
m
) 55. 0
at
2
' 54.0
53.0
13.0
52.0
51.0
12.0
5.0
wt.
7.0
5.0
6.0
% MgO
4.0
wt. S Hga
2.0
A.
0.0
7.0
6.0
5.0
caltenon
v
frac
a
4.0
I
+
+
+
I
7.0
7.0
7.0
8.0
5.0
wt.
X MgO
4.0
3.0
S
I
6.0
5
I
3.0
7.0
6.0
5 0
4.0
3.0
wt. % MgO
Fig. 1
Results of fractional crystallization model described
Asterisks = corrected Callahan data, plus signs = fractionation model.
in tex , presented in the form of oxide-oxide diagrams.
64.
composition
similar
to
83-18.
The
seperation of olivine and
plagioclase from this liquid evolves the liquid
direction
from
Callahan flow.
followed
by
by
the
It
direction
is
of
impossible
in
a
different
evolution represented by the
to
evolve
the
liquid
path
the Callahan flow samples in the ol-cpx-qtz ternary
fractionating
phases
consistent
with
the
phenocryst
assemblages observed in the flow.
Combined Fractional Crystallization and
Assimilation
of
a
Crustal Component by Parental Basaltic Magma
If assimilation
feasible
process,
associated
chapter
of
in
assimilation
be
considered
it
must
of
is
crystallization
is,
however,
solid
best
so
the
a
in
geologically
the context of
N.L.
intimately
crystals
treated
that,
as
a
Bowen
sort
"the solution
related
for
of
(1928,
the
to
most
corollary
the
part,
of
and as governed by the same general laws." Bowen
so-called
AFC
(combined
and fractional crystallization) model which will be
applied and discussed in this section.
between
be
crystallization.
was perhaps the first proponent of the
assimilation
as
describes the dependence as follows:
rock
separation
to
general
fractional
10)
solid
is
two
processes
which might be provided by the
is
The
obvious
connection
that assimilation requires heat
latent
liberated by fractional crystallization.
heat
of
crystallization
65.
As concluded
incapable
of
or
fractional
generating
Callahan flow.
whether
above,
the
The purpose
not
crystallization
compositional
of
this
alone
variation
section
is
to
of
is
the
determine
a small percentage of assimilation of a crustal
anatectic melt accompanying fractional crystallization (AFC) will
generate
a
closer
match
to
the
trends
demonstrated
by the
Callahan flow.
The
assumptions
behind
the
discussion
(after
Grove and Baker, 1984) are that as
following
AFC
model
employed
the parental basalt rises from its mantle source
continental
crust,
decreasing
density
surroundings
(Marsh
its
ascent
contrast
and
rate
the
overlying
slows, as a result of the
between
Kantha,
into
in
the
1978).
diapir
and
its
At some point in its
ascent history, the magma body begins to interact with the crust.
The basalt must undergo fractional crystallization to supply both
the heat required to raise the
latent
heat
necessary
for
crust
melting.
to
its
The
solidus
AFC
and
the
calculation is
performed by adding the assimilant to the liquid while solids are
removed
through
specified in the
parent,
the
fractionation.
calculation
composition
The
include
variables
the
which
composition
of
the
of the assimilant, the proportions and
compositions of the fractionating phases, and the ratio
of assimilation to rate of fractional crystallization.
AFC Model
must be
of
rate
j~
66.
The initial liquid is chosen as 83-18, one of the most mafic
samples
in the Callahan data set.
The assimilant composition is
chosen as 902Ma, a silicic inclusion collected near Cinder
(902Ma).
The
fractionating phases are constrained by the phase
relations for the compositions of the Callahan flow
by
the
observed
phenocryst
assemblages.
suggest that liquids which plot
volume
when
projected
in
through
the
olivine
with
Olivine
and
primary
plagioclase.
should
be
and
samples.
are
also
flow
liquids
are
but
are
flow
liquids.
been
part
of
phenocrysts
Their
be accounted for by some additional process.
occurs in minor amounts (< 1%) but is
have
in the Callahan flow
not compositionally consistent with
having precipitated from Callahan
must
present
saturated
precipitated
Less Ca rich plagioclase and orthopyroxene
present
not
fractionating
magnesian
the fractionating assemblage.
modeling process.
olivine
+
assemblage
plagioclase.
to
held
The fractionating
fractionating assemblage it is
assimilation
is
rate
With
enough
possible
For the same
is
model,
modeled
specification
to
vary
the
of
rate
as
the
of
of fractional crystallization (r) and the
proportions of fractionating minerals in order to
Callahan data set.
to
constant throughout the
assemblage
the
origin
Clinopyroxene
reasons presented above in the fractional crystallization
the
phase
plagioclase
phenocrysts of the appropriate compositions to have
Callahan
and
plagioclase onto the ol-cpx-qtz
do,
olivine
liquids
The phase relations
ternary, as the Callahan flow liquids
from
Butte
best
fit
the
67.
A calculation based on modeling the process which
generated
the Callahan compositional variation as an AFC process is capable
of
approximating
trends
which
variations observed in the flow.
"best-fit"
model
calculation
are
resemble
the
major
The parameters of the so-called
summarized
in
Table
9.
This
employed
achieved.
model
was arrived at by an iterative approach in which the
proportions of olivine and plagioclase were adjusted
value
element
successively
increased
until
and
the
r
the best fit was
The so-called "best-fit" calculation requires a
ratio
of rate of assimilation to rate of fractionation of approximately
1.5:1, and a
fractionating
plagioclase.
was
3.25.
The
The
assimilating,
assemblage
of
29%
olivine
+
71%
Ca-Na exchange partition coefficient employed
fraction
of
fractionating
liquid
liquid
most
remaining
when
the
closely resembled the
most evolved sample in the Callahan flow was approximately 0.85.
The results of
provided
in
Fig.
the
so-called
17.
The model closely approximates the Si02
"best-fit"
and K20 trends of the Callahan flow.
with
decreasing
MgO,
calculation
are
A1203 decreases too rapidly
FeO does not decrease rapidly enough, and
CaO and Ti02 are slightly enriched with respect to
the
Callahan
trends.
An r value of 1.5:1 is required in particular to predict the
Ti02
variation
slightly
with
observed
decreasing
in
the Callahan flow.
MgO.
No
evidence
Ti02 decreases
exists
for
the
68.
Table 9.
Parameters for the Best-Fit AFC Calculation in Figure X.
Si02
Ti02
A1203
FeO
Parent:
52.71
0.99
17.96
Contaminant
74.50
0.13
13.22
Na20
K20
9.55
3.4
0.68
1.27
4.04
4.67
MgO
CaO
8.27
6.45
1.52
0.57
Proportions of Fractionating Assemblage:
Ratio of Rate of Assimilation to
Rate of Fractional Crystallization:
1.5:1
29% ol+ 71% plag
-- ~~1-~1~'"1~~ ^~a~p-~
69.
2.0
$- 1.0
5..
0
0.0
5.0
5.0
7.0
4.0
59.0
I
II
7.0
6.0
5.0
4
7.0
6.0
5.0
410
6.0
, 19.0
18.0
'o 55.0
9 54.0
.
53.0
-
a
52.0
51.0
7.0
6.0
wt.
5.0
4 .0
4.0
8.0
% MgO
wt. 6 MgO
2.0
-
I
INN
'N,
CllIahan
0
AFC
+
0.0
7.0
6.0
5.0
4.0
I
,
.
I
i
I
.
7.0
6.0
wt.
4.0
Mg0
7.0
6.0
5.0
4.0
wt. X MgO
Fig. 17 Results of combined fractional crystallization and assimilation (AFC) model described in text, presented in the form of oxide-oxide diagrams. See figure 16 for symbol descriptions.
70.
fractionation
of
any
phases
for which Ti02 behaves compatibly
during the evolution of the flow.
than
or
equal
counter-balance
Elevated
r
values
(greater
to approximately 1.5:1) are required in order to
the
incompatible
behavior
predicted
by
the
fractionating phases.
Na20 is difficult to fit.
analytical
errors associated with its measurement.
above in the Major
analyses
This may be a consequence of
done
Element
Chemistry
section,
(Gladney
As discussed
the
Na20
wt.%
at the University of Massachusetts (U. Mass.) on
the USGS standard BCR-1 differ significantly
value
from
the
accepted
and Burns, 1983), as do the analyses of the same
standard done at the USGS facility in Lakewood, Colorado.
correction
factor
is
applied
approximately
5%
increase
for
Mass.
a
samples
the USGS samples) the Na20
variation with decreasing MgO is better constrained to
variation.
If
to the Na20 wt.% values for both
data sets (approximately 5% decrease for the U.
and
the
a
linear
It is apparent from the corrected data set for Na20,
however, that considerable variation exists in the
for a given M0O content.
Na20
content
The model closely predicts the slope of
Na20 variation with decreasing MgO.
The model presented in Fig.
Si02,
K20,
closer
fit
Mg0O, Ti02, Na20, and CaO, than for A1203 and FeO.
major uncertainty in modeling the
chemical
17 provides a
variations
of
the
process
Callahan
which
flow
as
generated
for
A
the
an AFC process
71.
surrounds the choice
presented
in
this
of
assimilant.
discussion
the
In
the
AFC
calculation
assimilant is modeled after
902Ma, a silicic inclusion collected near the proposed
the
flow
(Fig.
2).
If
the
for
Callahan flow evolved by an AFC
process, the difficulties in modeling A1203 and
above
vent
FeO
encountered
may imply that 902Ma is not representative of the material
assimilated by the Callahan flow.
The AFC calculations of
Baker, pers.
DePaolo
(1981,
modified
by
comm., 1985) have been applied to the Callahan flow
samples in order to fine tune the major element results.
emphasized
that
only
available,
thus
the
Results
M.B.
of
this
limited
trace
conclusions
are
It
element data is currently
preliminary
in
nature.
modified AFC program are provided in Fig.
and the parameters chosen for the calculations are summarized
Table
most
10.
The
evolved
contaminant
18,
in
parent lava was chosen as sample SM-75-213, the
lava
was
is
as
SM-75-137
modeled
after
(Mertzman,
902Ma.
The
1977)
bulk
and
the
partition
coefficients employed were calculated based on phenocryst- matrix
partition coefficients for basalts and andesites provided by Gill
(1981, p.
200) and the fractionating assemblage was that used to
model the major elements above.
The results of the trace element modeling shown in Fig.
are
in
general
18
agreement with the results of the major element
modeling presented above.
Solutions for
incompatible
elements,
72.
3. 0
Rb
K20
Bo
Zr
0(
]
aI]A
9
2.0
GJJX
(DX
(D[
(D
(D [IN
1.0
0.
(D (D
40]
CD[
ID
,A I
oil
0.0
El
.&XCD
121
0.2
0. 4
I
4<S I 1
0. 6
I
ID
0. B
1.0
Fig. 18 Results of incompatible trace element AFC model described in text.
Error bars indicate lateral extremes in possible solutions associated with
each curve.
73.
Table 10.
Parameters used in the AFC calculation in figure 20.
---------------------------------------------------element bulk
fraction
concentration (ppm) in:
fraction
D
error
most
most
possble
error
mafic evolved contaminant
-'------------------------------------------------Rb
0.05
0.01
15
43
130
0.1
625
0.1
225
375
0.10
Ba
0.12
0.1
250
150
200
0.10
Zr
0.01
0.1
4.66
0.62
1.59
0.08
0.01
K20
(wt. %)
74.
i.e.
Rb, Ba, Zr, and K20 converge at r values of 1 and greater.
The closest match appears to be at r=1.5.
remaining
(F)
suggested
The fraction of liquid
by the solutions is approximately 0.8.
Modeling attempts with compatible trace elements (i.e.
Sr)
and
REE were inconclusive because the variations in such elements are
slight.
The 87Sr/86Sr ratios determined for samples
in
the
Callahan
the
from
A constant 87Sr/86Sr ratio is consistent
derivation
an
of
isotopically
differentiation
of
of
an
homogenous
the
element
source,
2)
the
subsequent
parental liquid of the Callahan flow by
like
not
strontium,
material which is isotopically similar to
and/or
with
the parental liquid of the Callahan flow
fractional crystallization, which would
ratio
inclusions
flow do not exhibit distinct variation, beyond
analytical error.
1)
and
3)
the
vary
the
isotopic
the assimilation of
parental
liquid,
4) the assimilation of material which, although it may be
enriched in 87Sr/86Sr with respect to the Callahan flow
has very little Sr in it.
87Sr/86Sr ratios for the Callahan flow
are plotted against ppm Sr in Fig.
fractional
samples,
crystallization
and
13.
If a process of combined
assimilation is responsible for
the compositional variation in the Callahan flow,
an
assimilant
similar to that represented by the silicic inclusion 83-37a would
be isotopically
consistent,
as
its
87Sr/86Sr
ratio
slightly enriched with respect to that of the flow.
is
only
75.
The existing trace element and radiogenic isotope ratio data
suggest
that
the
single
crystallization process
variation
may
occurred.
While the
slight,
be
further
isotopically
history.
which
more
study
sensitivity.
and
assimilation and fractional
approximates
simplistic
variations
may
distinct
Nd
combined
than
Sr
expose
the
action
studies
actually
isotope
during
data
are
of two or more
the
may
element
which
the
isotopic
major
that
in
contaminants
Pb
the
assimilation
provide
Scatter in the trace element data (Fig.
greater
11) may be
a function of a multi-stage assimilation process or the result of
replenishing the magma chamber in which the Callahan flow evolved
with
fresh
parental
magma
(OHara,
1978).
Trace element and
radiogenic isotope work on samples collected within
of
a
detailed
flow
stratigraphy
the
context
within the Callahan flow may
provide the neccessary control required to
unravel
the
various
trends.
The major and trace element and radiogenic isotope data
with the evolution of the Callahan flow by a combined
consistent
assimilation and fractionation process.
suggest
that
fractional
ratios
of
crystallization
appropriate,
and
the
rates
of
of
1:1
87Sr/86Sr
The trace element models
assimilation
or
ratios
greater
require
assimilation process was operative during the
Callahan
are
flow,
the
assimilated
to
rates
might
that
evolution
if
of
of
be
an
the
material must have been either
isotopically similar to the assimilating liquid, or extremely low
76.
in
Sr such that it would have very minor effects on the isotopic
even
liquid,
assimilating
the
ratio of
with
rates
high
of
assimilation to fractional crystallization.
Mixing of Basalt and Rhyolite
be
to
mechanism
differentiation
final
and
fourth
The
considered is the mixing of a basaltic magma and a silicic magma.
to
Textural evidence exists for magma mixing in the intermediate
Medicine Lake Shield Volcano (e.g.
the
at
lavas
silicic
more
the
Dacite
Mixed
Older
Eichelberger, 1975;
The Callahan
phenocrysts
plagioclase
textures.
requiring
The
any
contain
postulated
distribution
of
disequilibrium
abundant
however,
glass,
groundmass
bimodal
a
exhibit
and
1941;
Grove et al., 1982).
Anderson, 1976;
samples
(Anderson,
recognized
have
investigators
previous
1982), which
Grove,
and
Gerlach
of
and
1975
Eichelberger,
Glass Mountain dacite, Anderson, 1941;
homogenous,
is
mixing process to have been thorough.
The rectilinear nature of the variations of the major elements on
oxide-oxide
diagrams
(Fig.
10) might logically be explained as
the result of some mixing process
individual
components
in
which
the
abundances
in a given sample are direct functions of
Gerlach and
the proportions of two end-members (McBirney, 1980).
Grove
(1982)
of
conclude
that
evidence
exists
continuous compositional spectrum of magmas in the
for
mixing
case
of
a
the
77.
basaltic andesite portion of the calc-alkaline series at Medicine
Lake.
If
magma
generated
is
mixing
is
considered
as
the
mechanism
which
the near linear variations on oxide-oxide diagrams, it
neccessary
to
end-members.
speculate
Gerlach
and
on
the
Grove
compositions
(1982)
of
the
conclude that for the
phenocrysts present in the Callahan flow, the mafic end-member is
characterized
by
XFe=0.38
and
XCa=0.86-0.88
end-member by XFe=0.68 and XCa=0.38.
calculated
using
the
and
the silicic
The XFe and XCa values were
two populations of phenocrysts present in
the Callahan flow to estimate the compositions of the
liquids,
and
the
results
of
melting experiments performed on
Medicine Lake volcanics (Grove et al.,
distribution
coefficients.
The
similar to
the
HAB
which
emplacement
of
the
Callahan
end-member
were
1982) to estimate exchange
postulated mafic end-member is
emplaced
flow
just
(Grove
prior
to
the
et al., 1982).
The
nature and origin of the silicic component is more
difficult
to
constrain.
McBirney
(1980)
compositionally
zoned,
suggests
that
calc-alkaline
density
magmas
during cooling and crystallization of basalt at
The
effect
of
the
compositional
change
stratified,
can
be generated
shallow
depths.
resulting
from
crystallization near the walls of a magma chamber will more
than
offset the increase in density caused by cooling, creating a zone
78.
of light,
silicic
evolved
liquid
liquid
may
along
collect
at
the
walls.
the
top of the magma chamber.
Eventually
this
Large eruptions tapping this magma chamber could bring magma from
two
or more zones when a large part of the reservoir is drained,
the force of eruption causing partial to total
stratified,
could
compositionally
provide
end-members
the
for
zoned
continuous
mixing.
Density
calc-alkaline magma chambers
compositional
spectrum
mixing suggested by Gerlach and Grove (1982) as
needed for the evolution of basaltic andesites at Medicine
Spera
et
al.
Lake.
(1984), however, maintain that the compositional
change in the liquid
walls
of
resulting
from
crystallization
near
the
of a magma chamber, assuming that the chamber behaves as a
closed system, is not sufficient to offset
structure
imposed
the
descending
flow
at the walls by the thermally induced density
contrasts.
Only rapidly diffusing components such as H20 in low-
viscosity,
basaltic
chambers
may show so-called countercurrent
flow and rise to the top of the chamber.
diffuse
more
Chemical species
which
slowly in magmatic liquids such as silica will not
show countercurrent flow.
An additional mechanism for generating the
in
the
observed
Callahan flow is the mixing of two genetically unrelated
magmas, one mafic and one silicic.
viscosities
The differing
densities
and
of the two magmas, however, make mixing difficult to
envision (Yoder, 1973).
hotter
trends
denser
If a cooler, lighter
magma
overlies
a
magma, the two will turn over and mix as a single
79.
convective unit
temperature
only
the
if
difference
driving
offsets
force
the
imparted
effect
of
the
by
compositional
differences and causes the Rayleigh number to exceed its critical
value
(McBirney,
the viscosity
temperature
1980).
In the case of a basalt and rhyolite,
difference
is
differences,
large
so
that
even
with
large
convection would only occur within the
individual layers and mixing will not result.
The impediment to mixing of
also
applies
in
the
case
compositionally zoned magma
compositionally
thermally
cooler,
more
of
silicic
currents
liquid
and
generate
the
heterogeneities
Callahan
flow.
genetically
both
unrelated
cases,
magmas,
need
to
either
the
the
mixing
or
stratified,
If
the
dominate
the
a
less
intermingle
observed
the
temperature
differences
of
required
is
Possible mechanisms for mixing include the tapping of
during
eruption
basaltic liquid.
or
the
invasion
In such cases,
of
a
however,
silicic
the
lavas cited above are the most likely products.
Mixing Models
in
mixing
of
to
the
two
two liquids
related by fractional crystallization, a driving force
above
dense,
collects at the top of a
liquids
In
viscosity
above.
currents
basaltic magma chamber, these
compositional
in
density
discussed
buoyancy
downwelling
viscous
contrasts
the
chamber
generated
generated
large
over
and
for
mixing.
a
chamber
chamber by a
partially
mixed
80.
Two
different
involved
mixing
considering
83-18
83-31 (one of the most
boundary
layer
models
were
employed.
a
silicic
fractionation
liquid
similar
first
(one of the most mafic samples) and
samples)
process.
to
be
to
83-18
in
related
by
a
In the scheme discussed
above from McBirney (1980) a liquid similar to
above
The
a
83-31
density
would
lie
stratified,
compositionally zoned magma chamber.
The
mix
to erruption to produce the
in
varying
proportions
prior
two
end-members
then
chemically variable flows which comprise the Callahan flow.
The
second involved mixing a mafic end-member similar to 83-18 with a
silicic liquid, in this case modeled
after
902Ma
(Table
4)
a
silicic inclusion found near the proposed vent of the flow.
The results of the first mixing calculation performed
one
of the most mafic samples and one of the most silicic sample
of the Callahan flow as end-members for mixing are
Fig.
using
19
on
oxide-oxide
presented
in
diagrams (computer model employed was
after Le Maitre, 1981, modified by Baker, 1985).
The calculation
simply provides a best fit for a given composition by varying the
proportions of specified
exhibited
end-members.
Given
that
the
trends
by the flow are linear and that the end-members chosen
for mixing lie on the linear trends it comes as no surprise
that
the model closely resembles the data.
The second mixing model in which a mafic end-member such
83-18
and
as
a rhyolite mix to generate the chemical variations of
81.
2.0
1.5
ma
B
ago
7.0
F
Sa
4%
aIa
0.5
0.0
6.0
70
4 .0
5.0
59 0
5.0
i
F.0
I
i
d=
i
.(4.0
5.0
5.0
18.0
a
AI
58 0
57.0
-
17.5
tl
56.0
'
$B
-
4
a
55.0
-
*' 54.0
a+
-a
S
53.0
I
I
52 0
51.0
I
I
7.0
I
6 0
ut.
,
5.0
%
16 0
4.0
7.0
5.0
6.0
Og0
wt.
4.0
% Mg0
aa
I
00
al"
a
+
4.0
5.0
6.0
7.0
cellahan
aIrang
10.0
a
aa
ma
6.0
a a
*
7 .0.
a
a
I
Ba
8.0
a
D
m
I
i
m
7.0 7.0
8.0
wt.
5.0
Mgo0
4.0
7.0
6.0
wt.
5.0
I
40
MgO
Fig. 19 Results of first mixing model described in text, presented
in the form of oxide-oxide diagrams. Open circles = corrected Callahan data (see AFC modeling discussion), plus signs = mixing
model.
82.
the Callahan flow is evaluated in Fig.
20.
In this example
the
silicic end-member candidates include older rhyolites from around
the Callahan flow and a silicic inclusion from within
the
flow.
It is obvious that mixing between a liquid similar to the silicic
inclusion and a liquid similar to 83-18
trend
demonstrated
diagram.
well.
by
the
Callahan
will
flow
This model fails for the remaining
It
is
impossible
to
rule
not
generate
the
on an Si02 vs.
MgO
major
elements
as
out, however, the fact that
mixing might have occurred with an appropriate silicic liquid and
that
remains
of
this
silicic
liquid
simply
have
not
been
preserved or observed in the flow.
The first scenario
related
involving
the
mixing
of
two
one to the other by a fractional crystallization process
is more geologically feasible than
mixing of basalt and rhyolite.
chosen for mixing
viscosity
and
temperature.
scenario
involving
however,
in
other
terms
density,
of
The limitations on probable mixing
This
case
the disequilibrium phenocrysts present in the
compositional end-members of the Callahan flow (figs.
The
the
In the first case the two liquids
each
resemble
the
discussed above, therefore, would not be as extreme.
ignores,
liquids
3 and
6).
disequilibrium plagioclases and olivines suggest that 83-18,
the mafic end-member, and
themselves
the
products
83-31,
the
silicic
end-member,
are
of the process(es) which generated the
variations observed, requiring the
actual
end
members
of
mixing process to be more disimilar (Gerlach and Grove, 1982).
the
83.
80.0
call ahan
0
"
rhy inc
0
old rhy
ID
70. 0
60. 0
50.0
7.0
L
6.0
5.0
wt.
4.0
%
3.0
2.0
1.0
0.0
MgO
Fig. 20 Evaluation of second mixing model described in text. Wt. % Si02
is plotted against MgO for the Callahan samples (triangles), a rhyolite
inclusion (circle), and older rhyolites from around the flow (boxes).
84.
In conclusion, the linear trends demonstrated by
elements
end-members.
difficult
mixing
of
appropriate
The different rheological properties of basalts and
however,
makes
thorough
mixing
to envision (McBirney, 1980).
between
increases.
the
two
As two liquids approach
similar viscosities and densities, the ease with which
mix
major
of the Callahan flow on oxide-oxide diagrams can easily
be explained as having resulted from the
rhyolites,
the
they
can
While 83-18 and 83-31 are not very different in
terms of such properties, disequilibrium
phenocryst
assemblages
in the two end-member samples suggest that the samples themselves
are products of the mixing process.
85.
Implications of the AFC Model
Of the four petrogenetic models discussed
section,
the
AFC
(combined
crystallization) model most
in
assimilation
successfully
the
and
reproduces
element variations observed in the Callahan flow.
other three models, however, were involved in
the
following
ways:
previous
fractional
the
major
Aspects of the
the
evolution
in
1) partial melting processes generated the
parental HAB, 2) fractional crystallization, possibly at elevated
pressures,
played
an
important
role
in
liquid, and 4) although the mixing of two
magmas
problably
did
the evolution of the
genetically
unrelated
not have a role in the differentiation of
the Callahan flow, mixing processes may have operated within
the
magmatic plumbing system of the volcano.
The results of the
AFC
model
which
best
fit
the
major
element trends suggest that the Callahan flow represents a series
of
liquids
experienced
related
of
rate
by
of
fractional
assimilation
crystallization (r) of 1.5:1.
at
the
point
crystallization
to
rate
The fraction of
of
which
fractional
liquid
remaining
where the assimilating, fractionating liquid most
closely resembles the average most silicic sample of the flow
approximately
modeling
0.8
purposes
composition.
The
to
was
0.85.
similar
fractionating
The
to
parental
83-18
in
is
liquid
used for
major
element
assemblage used was consistent
with the phase relations predicted within the ol-cpx-qtz
ternary
d
86.
and the phenocryst assemblages observed in the flow.
The ratio of rate of
crystallization
assimilation
calculation
conditions
rate
of
fractional
of 1.5:1 suggested by the model is high relative
to the amount predicted by
balance
to
a
(1:3)
straight
forward
appropriate
(Grove and Baker,
1984).
thermal
energy
for upper crustal level
The
required
energy
for
melting crust depends on the temperature differential between the
intruded basalt and the
ambient
intrusion
and
on
depth
the
intrusion
of
temperature
at
the
level
the latent heat of the melting crust.
the
increases,
ambient
of
As the
temperature
increases and the amount of heat required to melt the surrounding
walls of a magma chamber decreases.
The AFC process is poorly understood.
have
been
(Daly,
proposed
or
1903)
are
Some mechanisms which
stoping of enitire blocks by the magma
selective
leaching
of
the
more
mobile
consitutents from the wall rocks by the magma (Watson, 1982).
the
following
occurred
discussion,
it
the
amount
allowing for elevated
temperature
considerations.
of
heat
needed
assimilation
r
values,
is
a
One
way
to
for the process, thereby
to
increase
the
ambient
over and above the geothermal gradient as the result
A simple calculation
of long-term magmatic activity in the area.
assuming
that
in bulk, allowing rough constraints to be placed on the
AFC model based on thermal budget
decrease
assumed
is
In
specific
heat of 0.33 cal/q
0
C demonstrates that in
87.
order to achieve an r of
1.5:1
at
upper
crustal
levels,
ambient
temperature
925 oC.
Assume that the basalt intrudes at 1200 oC and that
solidus
of the assimilated material is 1000 oC.
the
must be raised from approximately 200 oC to
the
Estimates based
on thermochemical data (Hon and Weill, 1982) indicate that latent
heats
of
melting
respectively.
cal.
for
basalt
The crystallization of 1 g of basalt liberates 100
The melting of 1.5 g of assimilant requires 75 cal, leaving
25 cal to put towards raising
assimilant
to
its
solidus
assumed above,
.33
cal
assimilant 1 oC.
of
and granite are 100 and 50 cal/g
assimilant
required
the
ambient
temperature.
are
required
temperature
of
the
For the specific heat
to
raise
our
1
g
of
The 25 cal then are available to raise this 1 g
approximately
75oC.
The
initial
temperature
for r to be 1:1.5 at upper crustal levels, then, is 925
oC, implying that the long-term magmatic
activity
of
the 'area
must increase the ambient temperature from 200 oC as predicted by
the geothermal gradients to 925oC.
extreme,
and
the
postulation
of
supported by an anomolously high
Unfortunately,
Lake.
little
is
An increase
of
725oC
seems
such an increase needs to be
heat
flow
over
the
volcano.
known about the heat flow at Medicine
Moving the entire scenario to lower depths
increases
the
ambient temperature, decreasing the temperature differential.
An additional way to approach the AFC process
return
to
the
existence
of
a
would
be
to
boundary layer in a convecting
basaltic magma chamber (McBirney, 1980;
Spera
et
al.,
1984).
88.
The
boundary
thermal
layer
gradient
increased
is
generated by both the compositional and
imposed
at
crystallization.
the
margin
of
the
chamber
The interaction of the compositional
and thermal effects in the boundary layer is not agreed
the
literature
vs.
Spera et al., 1981, 1982, 1984).
considered
(e.g.
alone
boundary)
the
crystallization
no
along
the
If
the
chamber
If
influx
cooler
the
is
thermal
along
boundary
margin
denser than the interior of
result.
upon
in
McBirney, 1980, and Huppert et al., 1984
(i.e.
then
by
effects
are
the magma-wall rock
layer
generated
by
of the magma chamber will be
chamber
and
isothermal
downwelling
(i.e.
will
no temperature
gradient from the interior of the chamber out towards the margin)
then
the more evolved boundary layer will be less dense than the
interior of the chamber and
general
case
involves
the
upwelling
interaction
gradient and a compositional gradient.
al.
will
result.
of
The
more
both a temperature
In this
case,
Spera
et
(1984) demonstrated that if the chamber behaves as a closed
system,
the
thermal
compositional
downwelling
upwelling
effect
effect
dominates
resulting
over
the
in a net downwelling
effect and differentiation of a magma by boundary layer processes
will
not occur.
and flux occurs
(McBirney,
1980;
If the magma chamber behaves as an open system,
across
the
wall
Huppert et al.,
rock-magma
1984, Spera et al.,
the resulting compositional upwelling
thermal
chamber
effect
can
boundary
1984) then
dominate
the
gradient downwelling effect and boundary layer processes
will occur to differentiate the magma.
Assimilation of at
least
89.
some
component
of
chamber, then,
the wall rock into the boundary layer of the
is
necessary
for
the
compositional
upwelling
effect to dominant the thermal downwelling effect.
This discussion can be taken one step further.
layer
The boundary
is the margin between the convecting basaltic magma in the
interior of the magma chamber and the wall rock.
boundary
layer
did
not
convect,
that
it
Assume that the
was
a
thermally
conductive margin between the convecting interior and the
host
rock.
Then
further
assume
that
the assimilation which
enables the boundary to behave bouyantly is
boundary
If
the
AFC
process
is
layer of the magma chamber (Fig.
of assimilating 1.5 g of material for every
more
constrained
to
the
layer, that the hot interior does not "see" the effects
of assimilation.
boundary
cooler
easily
handled.
The
constrained
to
the
21), the difficulties
g
fractionated
are
Callahan flow is interpreted as the
result of tapping the assimilating, fractionating boundary
layer
liquids of a HAB magma chamber.
Pyroxene phase relations in the Callahan flow
this
imposed
margin
and
Orthopyroxene
isolation
the
is
that
between the assimilating, fractionating
fractionating
the
suggest
interior
is
not
artifical.
phenocryst pyroxene phase in the Callahan
flow, interpreted as the margin of the chamber, and augite occurs
in
the
basaltic inclusions of the flow which can be inferred to
be samples of the interior of the chamber.
If
the
interior
of
~isa~~-
I
90.
/
\buoyancy
assimilati,
crystallization
I
composytional
and thermal
boundary layer
Fig. 21
Highly schematic sketch of an assimilating,
fractionating basaltic magma chamber.
chamber is approximately 2-3 km.
Width of the
91.
the
chamber
is
experiencing a typical HAB crystallization path
then augite is the first pyroxene to crystallize.
should
however,
not
crystallize
from
a
HAB parent until the
latest stages of fractional crystallization.
the
Orthopyroxene,
If
the
margin
of
chamber is permitted to assimilate then orthopyroxene can be
stabilized as the phenocryst phase in the Callahan flow.
Assimilation in the boundary
source
for
the
liquids.
would
could
As
not
previously
discussed,
found
compositions,
in
however,
intermediate
provide
a
the
are
compositions
in Burnt lava flow.
the
andesine
have crystallized from the Callahan flow
Low-Ca plagioclase phenocrysts
inclusions
also
andesine plagioclase phenocrysts which occur in
every Callahan sample.
phenocrysts
layer
occur
Callahan
generally
in
flow.
An-15
the
The
to
silicic
phenocryst
An-20.
More
occur in the silicic inclusions found
The Burnt lava
flow
inclusions
inclusions
are partially melted and contain resorbed plagioclase phenocrysts
similar in composition
to
the
andesine
grains
found
in
the
chamber,
the
Callahan flow.
In terms of the thermal budget of
the
magma
interior of the magma chamber fractionates without perceiving the
effects
of
assimilation,
fractionated.
margins
combined
of
This
the
energy
chamber,
liberating
100
cal
per
each
gram
is transferred by convection to the
providing
the
neccessary
heat
for
assimilation and fractional crystallization to occur in
92.
elevated proportions.
flow
In an approximate manner, if the
Callahan
is interpreted as a boundary layer for a HAB magma chamber,
and if one assumes further that it represents the entire boundary
layer,
then
it is possible to constrain the minimum size of the
chamber.
The following calculation attempts to infer
of
the
chamber.
magma
volume
chamber.
for
the
boundary
Thermal
boundary
layer
of 10s of m (Spera et al.,
1982).
of
this
thicknesses were
determined for anhydrous rhyolite magma chambers
order
dimensions
The calculation assumes that the Callahan flow
represents a minimum
postulated
the
to
be
on
the
Assuming a thickness of
10 m and 100 m, respectively, a simple calculation
provides
the
total
chamber.
To
minimum
volume
of
the
postulated
magma
simplify the geometry, the entire mass is contained in a box.
the
dimension
of the box is R and the thickness of the boundary
layer is r then the
(R-r)33.
volume
of
the
boundary
layer
is
ER3
-
Setting this equal to the estimated volume of the flow
and substituting 10 m and 100 m for r gives yields R
and
If
1.68 km respectively.
=
5.17
km
The total volume of the magma chamber
is 138 km3 and 4.74 km3 respectively.
If the shape of the
magma
is assumed to be cylindrical for simplicity's sake, with
chamber
a height of 5 km, then the radius is approximately 3 km and .5 km
respectively.
These
are
minimum sizes based on the assumption
that the Callahan flow represents
layer
of
a
convecting
basaltic
the
entire
magma
thermal
chamber,
boundary
and that the
93.
boundary layer thicknesses are 10 and 100 m respectively.
A simplistic heat balance check can be applied to
ratio
of
of assimilation to rate of crystallization to be 1.5:1.
If
permit
appropriate heat to the boundary layer to
the
the thickness of the boundary layer is assumed to be 10
the
size
The magma chamber has to be big enough to supply the
estimates.
rate
the
heat
then
15
137 km3 = 137x0
balance check proceeds as follows:
m,
cm3, and assuming a density for the magma of 2.7 g/cm3, converts
19
)t
g material. If 100% of this crystallizes, 3.70x10
to 3.70xl0
cal are liberated, and if all of it goes to assimilation, at
rate
of
314 cal/g (energy needed to first raise 1 g from 200 to
1.18x10
1000 oC and then to melt it),
g
material
of
.8 km3 of material in the Callahan flow was generated by
the
AFC
process
(= to 1.5(20%)
to
according
above,
modeled
then
it
consist
should
material).
fractionated
the
model
was
The
assimilated
mass
approximately .3 x .8 or .24 km3.
100% crystallization is capable of assimilating .5 km3, or
required
amount
magma
of
70% fractionated liquid + 30% assimilated material
approximately
the
be
km3.
cm3 or .49
material of 2.4 g/cm3 converts this to 4.91x10
the
can
density for the assimilated
average
an
Assuming
assimilated.
If
the
chamber
would
by
have
the model.
been
twice
50% crystallization of the
to
sufficient
permit
the
assimilation of 1.5 g of material for every g fractionated in the
margin.
thickness
calculation
similar
A
of
100
m
requires
assuming
133%
of
a
the
boundary
magma
layer
chamber to
94.
crystallize in order to permit
silicic
material
for
the
assimilation
of
1.5
g
of
every g crystallized in the interior.
In
other words, the thermal budget calculation suggests that if
Callahan
flow
the
represents the boundary layer of a basaltic magma
chamber, and if the rate of assimilation to
crystallization
in
the
rate
of
fractional
margin was 1.5:1, then the thickness of
the boundary layer must have been less than 100 m.
In
conclusion,
material
for
every
assimilation than is
process
in
assumptions
convecting
a
although
assimilation
of
1.5
g
of
g fractionated calls for a higher degree of
expected,
geologically
about
the
the
nature
it
is
possible
feasible
of
the
to
manner.
boundary
model
the
Simplifying
layer
of
a
basaltic magma chamber suggest that the Callahan flow
could represent the assimilating, fractionating margin, and place
limits
on
the
volume
of the chamber.
made for upper crustal level conditions
200
oC).
These calculations were
(ambient
chamber
=
assimilation occurred at greater depth, or if the
If
temperature gradient of the host rock along the 5
the
temperature
is
considered,
magma chamber is required.
km
height
of
less crystallization of the entire
95.
Conclusion
The
basaltic
andesite
Callahan
flow
at
Medicine
Lake
volcano, California, exhibits variations in major elements, trace
elements and radiogenic isotopic ratios which are best
by
a
explained
combined assimilation and fractional crystallization model
(AFC).
The
andesite
other
genesis
major
petrogenetic
processes
for
proposed
include partial melting of some combination of
the subducted oceanic plate and the
hydrated
mantle
above
it,
fractional' crystallization from a basaltic parent and the mixing
of basaltic and rhyolitic
processes
three
major
specific
contributed
element
Although
magmas.
and
to
aspects
of
these
the evolution of the flow, the
variations
element
trace
are
reproduced most successfully by the AFC model, in which the ratio
of rate of assimilation to rate of fractional crystallization
is
1.5:1.
The phenocryst assemblages present in the Callahan flow
consistent
with
an
AFC model.
The two sources for the bimodal
were
distribution of plagioclase phenocrysts
augite
the
interior and the assimilated material.
basaltic
in
predicted
the
basaltic
fractionation
is
inclusions
path
are
for
a
fractionating
The presence of
consistent
with
the
HAB, and the occurence of
orthopyroxene in the more silicic samples is a result of the high
degree of contamination experienced in the margin of the chamber.
I^____~m~CWi_
I~^
96.
The assimilation rate of 1.5 g of silicic material for every
g
of basalt fractionated is higher than would be anticipated for
simple models of uppper crustal level conditions (1:3).
the
combined
assimilation
and
fractionation
Modeling
process
in
the
compositional, thermal boundary layer of a basaltic magma chamber
permits
a
higher degree of assimilation.
If the eruption which
emplaced the Callahan flow tapped the entire
its
magma
chamber
then
the
boundary
layer
of
volume of the flow constrains the
minimum volume of the magma chamber.
Rough
assuming
10 m suggest that the magma
a
boundary
thickness
of
chamber associated with the Callahan flow,
cylinder,
was
5
km
in
height
then
the
magma
if
based
simplified
with a radius of 3 km.
thermal, compositional boundary layer is
thickness
estimates
modeled
as
100
on
as
a
If the
m
in
chamber associated with the flow, if
simplified as a cylinder of 5 km height, had a radius of
.5
km.
Thermal energy budget constraints estimate that in the first case
approximately 50% of the basaltic liquid fractionated in order to
permit
an
r
value of 1.5:1 in the boundary layer.
The thermal
energy budget also precludes a boundary layer thickness of 100
(for
the
assumed
geometry)
as the associated magma chamber is
then too small to generate the required amount
margin.
ambient
predicted
m
of
heat
at
the
If long-term magmatic activity in the area elevated the
temperature
at
the
depth
of
intrusion
above
that
by the geotherm, a lower percentage of crystallization
within the chamber is required.
~II~I~
_~I~_~__XI_
97.
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~~-11
ii^XI--__II--L
-I*YII~
~-(I
F~I~--~L--_
_
11~ __ 11-1*1~ 11~-
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----r~~lill-P CL-rir-----^----^~
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