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. K D s 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
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