This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
• 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