Download Mantle & Crust

Silicate Earth
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Primitive mantle
Present-day mantle
Crust
Oceanic crust
Continental crust
Reservoir
Volume
(1027 cm3)
Mass
(1027g)
Mass %
Earth
Core
Mantle
Crust (continental)
Hydrosphere
Atmosphere
1.083
0.175
0.899
0.00842
0.00137
-
5.98
1.88
4.08
0.0236
0.00141
0.000005
100
31.5
68.1
0.4
0.024
0.00009
Evidence for mantle composition:
• Sampled by xenoliths, occasionally exposed by crustal
deformation
– Peridotite
– Eclogite
• Seismic velocities match both rocks
• Must melt to form basaltic magma
– Peridotite melting – up to about 40%
– Eclogite melting – must be close to 100%
3 types of primary basaltic magma
•At divergent plate margins (mid ocean ridges) – magma rises from asthenosphere decompression melting at low pressure - tholeiitic basalt
•At hot spot (intra-plate volcanoes) – magma rises from deep mantle - decompression
melting at high pressure - alkali basalt
•At convergent plate margins (volcanic arcs) – water added to the mantle from the
subducted lithosphere causes melting - flux melting - calc-alkaline basalt
Present day mantle convection
patterns are deduced from study
of seismic wave velocities
(profiles and tomography), and
plate tectonics.
Composition of mantle
layers/reservoirs are deduced
from studies of xenoliths and
mantle-derived basalts.
Present-day mantle differs from
primitive mantle because of
extraction of material through
magmatism and crust
formation, and recycling of
crustal material through
subduction.
Trace elements in mantle
• Primitive mantle – chondrites for refractory elements,
ingenious estimates for volatile elements, mainly based
on isotopic calculations
• Present day mantle –
– Analyze xenoliths – but these are extremely variable
– Infer compositions by looking at basalts and modeling
the melting process
Mantle derived basalts:
Mineral/melt partition
or distribution
coefficients – define
the ratio of an element
in a mineral compared
with a melt at chemical
equilibrium.
Used to infer source
composition.
KDs reflect the ability of
an element to enter the
structure of the
mineral.
Ionic radius
Mantle derived basalts:
Mantle source of MORB – must be depleted compared to primitive mantle
Mantle source of OIB – must be enriched compared to primitive mantle
Mineral abundances: 39% plagioclase, 12% quartz, 12% K-feldspar, 11%
pyroxene, 5% mica, 5% amphibole, 3 olivine, 5%clay, 2% carbonate, 8%
other
“Stack Models”
Rudnick and Fountain, 1995 – first thorough study of upper
and lower continental crust
Based on seismic velocities, lower crust can be gabbro,
granulite or amphibolite, but not eclogite
Trace element estimates from:
1) Rock averages from stack models
(gabbro, granite, limestone, etc.)
2) Materials that sample large areas of
the crust as the result of sedimentary
processes like shale and loess –
North American Shale Composite
(NASC)
3) Heat production – match abundances
of K, U, Th
Puzzle: Why is average continental crust
andesitic? Where is the complementary
gabbroic material? Lower crust is
insufficient.
Two processes influence composition of continental crust:
•Transfer of elements in “water” from the subducted plate
•Melting
Much of continental crust was formed in subduction zones
Basalts from subduction zones – island arc basalt