Silicate Earth

• Primitive mantle• Present-day mantle• Crust• Oceanic crust• Continental crust

Reservoir Volume Mass Mass %(1027 cm3) (1027g)

Earth 1.083 5.98 100Core 0.175 1.88 31.5Mantle 0.899 4.08 68.1Crust (continental) 0.00842 0.0236 0.4Hydrosphere 0.00137 0.00141 0.024Atmosphere - 0.000005 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 mantleMantle 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

Basalts from subduction zones – island arc basalt

Much of continental crust was formed in subduction zones