Silicate Earth

1. Silicate Earth • Primitive mantle • Present-day mantle • Crust • Oceanic crust • Continental crust Reservoir Volume Mass Mass % (1027 cm3)(1027g) Earth 1.083 5.98 100 Core 0.175 1.88 31.5 Mantle 0.899 4.08 68.1 Crust (continental) 0.00842 0.0236 0.4 Hydrosphere 0.00137 0.00141 0.024 Atmosphere - 0.000005 0.00009

4. 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%

6. 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

7. 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.

9. 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

10. Mantle derived basalts:

11. 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

12. 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

14. Mineral abundances: 39% plagioclase, 12% quartz, 12% K-feldspar, 11% pyroxene, 5% mica, 5% amphibole, 3 olivine, 5%clay, 2% carbonate, 8% other

15. “Stack Models”

16. 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

17. Trace element estimates from: • Rock averages from stack models (gabbro, granite, limestone, etc.) • Materials that sample large areas of the crust as the result of sedimentary processes like shale and loess – North American Shale Composite (NASC) • 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.

21. Two processes influence composition of continental crust: • Transfer of elements in “water” from the subducted plate • Melting

22. Basalts from subduction zones – island arc basalt Much of continental crust was formed in subduction zones