Lecture 4: MORB petrogenesis
Outline
1)
2)
3)
4)
Overview of igneous petrogenesis
Mid-Ocean Ridges – how are they characterized?
MORB – where and how do they form?
Geochemical variations in MORB (major elements,
trace elements and isotopic characteristics)
Igneous Petrogenesis
1.
2.
3.
4.
5.
6.
7.
Mid-ocean ridges
Continental rifts
Island Arcs
Active continental margins
Back-arc basins
Ocean Islands
Intraplate hotspot activity, carbonatites, or kimberlites
Mid-ocean ridges
Mid-ocean ridges produce ~ 21 km3 of lava per year
~60% of the earth’s surface is covered with oceanic crust
Mid-ocean Ridges
Spreading rate influences thermal structure, physical
structure, crustal thickness and amount of melting
Spreading rate and structure
Fast-spreading East Pacific Rise
• Thermal structure is warmer
• Crust is thicker, lithosphere is thinner
• Higher degrees of melting
• Sustained magma chambers and
volcanism
• Less compositional diversity
Slow-spreading Mid-Atlantic Ridge
• Thermal structure is cooler
• Crust is thinner, lithosphere is thicker
• lower degrees of melting
• Episodic volcanism
• Higher compositional diversity
The Axial Magma Chamber: original model
•
Semi-permanent
•
MORB magmas are produced by fractional
crystallization within the chamber
•
Periodic reinjection of fresh, primitive MORB
•
Dikes upward through extending/faulting roof
•
Crystallization at top and sides  successive
layers of gabbro (layer 3) “infinite onion”
•
Dense olivine and pyroxene crystals 
ultramafic cumulates (layer 4)
•
Moho?? Seismic vs. Petrologic
Figure 13.16. From Byran and Moore (1977)
Geol. Soc. Amer. Bull., 88, 556-570.
Hekinian et al. (1976)
Contr. Min. Pet. 58, 107.
A modern concept of the axial
magma chamber beneath a fastspreading ridge
After Perfit et al. (1994)
Geology, 22, 375-379.
Model for magma chamber beneath a slow-spreading
ridge, such as the Mid-Atlantic Ridge
•
Most of body well below the liquidus temperature, so convection and mixing is
far less likely than at fast ridges
•
numerous, small, ephemeral magma bodies occur at slow ridges
•
Slow ridges are generally less differentiated than fast ridges - no continuous
liquid lenses, so magmas entering the axial area are more likely to erupt
directly to the surface
2
Depth (km)
Rift Valley
4
6
Moho
Transition
zone
Gabbro
Mush
8
10
5
0
5
Distance (km)
10
After Sinton and Detrick (1992) J. Geophys. Res., 97, 197-216.
Oceanic Crust and Upper
Mantle Structure
1) Geophysical studies
2) Mantle xenoliths
3) Ophiolites: uplifted oceanic crust
+ upper mantle
Lithology and thickness of a typical
ophiolite sequence, based on
the Samial Ophiolite in Oman.
After
Boudier and Nicolas (1985) Earth Planet. Sci. Lett., 76, 84-92.
Rock types in the mantle
Peridotite is the dominant rock type of the Earth’s upper mantle
• Lherzolite: fertile unaltered mantle; mostly composed of olivine,
orthopyroxene (commonly enstatite), and clinopyroxene (diopside), and
have relatively high proportions of basaltic ingredients (garnet and
clinopyroxene).
• Dunite (mostly olivine) and Harzburgite (olivine + orthopyroxene) are
refractory residuum after basalt has been extracted by partial melting
• Wehrlite: mostly composed of olivine plus clinopyroxene.
wehrlite
lherzolite
Ocean Crust Geology
Modern and ancient pillow basalts
Glassy pillow rinds are used to infer
original melt compositions
P. Asimow
Magma: mixture of molten rock, gases and mineral phases,
produced by mantle melting
Mantle melts between ~800-1250ºC due to:
1) Increase in temperature
2) Decrease in pressure
3) Addition of volatile phases
Partial melting
Adiabatic rise of
mantle material with
no heat loss –
decompression
melting
Mid-Ocean Ridges
A model for mantle melting
• Several models are possible of how and where the melt is extracted
and what happens to it during transport
• This average melt is primary mid-ocean ridge basalt (MORB).
• Hot mantle starts melting at deeper depths, thus has a larger melt
triangle or area over which melting occurs than a cooler mantle
• Mantle rising nearer axis of plume traverses greater portion of
triangle and thus melts more extensively
Hot mantle
cool mantle
Asimow et al., 2004
Igneous rock classification by composition
• There are several classifications, of individual rocks or rock suites.
• By silica percentage:
%SiO2 Designation
% Dark Minerals
Designation
Example
rocks
>66
Acid
<40
Felsic
Granite, rhyolite
52-66 Intermediate
40-70
Intermediate
Diorite, andesite
45-52 Basic
70-90
Mafic
Gabbro, basalt
<45
Ultrabasic
>90
Ultramafic
Dunite, komatiite
(plagioclase)
The common crystallization sequence at
mid-ocean ridges is: olivine ( Mg-Cr
spinel), olivine + plagioclase ( Mg-Cr
spinel), olivine + plagioclase +
clinopyroxene
(clinopyroxene)
After Bowen (1915), A. J. Sci., and Morse (1994)
(olivine)
The major element
chemistry of MORBs
• MORBs are the product of fractional
crystallization, melt aggregation,
seawater interaction and crustal
contamination
• MgO contents are a good index for
fractional crystallization (typically,
more primitive melts have higher
MgO)
• Data is often “corrected” back to 8
wt% MgO to estimate primary melt
compositions and to compare data
sets
“Fenner-type” variation diagrams for basaltic glasses from
the Afar region of the MAR. From Stakes et al. (1984)
Increased fractional crystallization
Global systematics
•
The values of regionally-averaged Na8 (i.e., Na2O concentration
corrected to 8% MgO), Fe8, water depth above the ridge axis, and
crustal thickness show significant global correlations.
– Where Na8 is high, Fe8 is low
– Where Na8 is high, the ridges are deep
– Where Na8 is high, the crust is thin
3 .5
3 .0
Na8 is an incompatible
element, thus an indicator
of mean extent of melting.
2 .5
Fe8 is an indicator of mean
pressure of melting.
Deep ridges
N a 8 .0
2 .0
1 .5
Shallow ridges
6
7
8
9
Fe 8 .0
10
11
12
Axial depth is an indicator
of mantle temperature,
extent of melting, and
crustal thickness
combined – see slide #5
Synthesis of global systematics
•
•
The global correlation implies that extent of melting and pressure of
melting are positively correlated, on a global scale. This relates to the
mantle potential temperature.
If melting continues under the axis to the base of the crust everywhere,
then high potential temperature means: long melting column  high
mean extent of melting  low Na8 and high crustal thickness  shallow
axial depth; high mean pressure of melting  high Fe8. Cold mantle
yields the opposite.
Hot mantle
Cold mantle
sea level
axial depth
crust
mean
P
25%
20%
15%
10%
40%
F
mean
F
5%
35%
solidus 1.5 GPa
30%
F
mean
P
25%
20%
15%
10%
mean
F
5%
solidus 4.5 GPa
P. Asimow
Spider diagram of crust vs mantle
Workman and Hart, 2005
A modern concept of the axial
magma chamber beneath a fastspreading ridge
Figure 13-15. After Perfit et al.
(1994) Geology, 22, 375-379.
Generating enriched signatures in MORB
1)
2)
Low degrees of melting
Mantle source enrichment
•
•
•
N-MORB: normal MORB
T-MORB: transitional MORB
E-MORB: enriched MORB
Isotope systematics of MORB
Radiogenic isotope systems (Sr, Nd, Pb) are used to see mantle enrichments
due to relative compatibilities of radiogenic parents and daughters
87Sr, Rb is more incompatible than Sr so high 87Sr/86Sr ratios
e.g., 87Rb
indicate an enriched source
Compared to ocean islands and subduction zones, MORBs are relatively
homogeneous
Stable isotopes
• Like radiogenic isotopes, stable isotope can be used to trace source
enrichments and are not influenced by degrees of melting
• Oxygen, boron, helium and nitrogen isotopes show very little variability
in MORB, and are distinct from enriched OIB and subduction related
lavas
Manus
Macpherson et al., 2000
He isotopes:
3He
: key tracer of a primordial component
4He : representing a radiogenic component (U+Th decay)
3He
anomalies at ridges is evidence for degassing of primordial gases
from the earth
Typical 3He/4He ratios:
Crust : 0.01-0.05 RA
MORB : 8 ± 1 RA
Arcs: 5 - 8 RA
Hotspots: up to 37RA
Craig and Lupton (1981)