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Geochemical constraints on mantle structure and convection

Geochemical constraints on mantle structure and convection. ( For example……. ) Trace element fractionation during partial melting. Melts extracted from the mantle rise to the crust, carrying with them their “enrichment” in incompatible elements.

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Geochemical constraints on mantle structure and convection

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  1. Geochemical constraints on mantle structure and convection

  2. (For example…….) Trace element fractionation during partial melting • Melts extracted from the mantle rise to the crust, carrying with • them their “enrichment” in incompatible elements. • Continental crust becomes “incompatible element enriched”. • Mantle becomes “incompatible element depleted”. Mantle Melting product: >Rb/Sr <Sm/Nd >U,Th/Pb Ni Sm Region of partial melting Nd Co V Rb Pb Sr Melting residue: <Rb/Sr >Sm/Nd <U,Th/Pb Th U Incompatibles Compatibles Cr

  3. A simple mantle evolution (example of “petrogenetic tracing”…… Figure 9.13. Estimated Rb and Sr isotopic evolution of the Earth’s upper mantle, assuming a large-scale melting event producing granitic-type continental rocks at 3.0 Ga b.p After Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer.

  4. OIB enriched in incompatibles but different OIBs are different MORB formed from already depleted mantle

  5. Mixing: MORB--OIB--CC?? EM-2 enriched (cont. seds) Regions depleted in incompatibles have low 87/86 but high 143/144

  6. Apparent age is 2 Gyr No simple trend -- CC is in middle -- therefore not back-mixing of CC into mantle

  7. Note convergence of HIMU and MORB at FOZO

  8. Many hotspots have limited range of isotopic compositions EM1 and EM2 are concentrated south of equator -- DUPAL anomaly Indian ocean MORB is different from Atlantic and Pacific

  9. In 3-D space…. it is a zoo(!) of mantle reservoirs)

  10. (Other) sources of mantle compositional heterogeneity:

  11. Other isotopic systems that contribute to our understanding of mantle reservoirs and dynamics Noble gases – e.g., He (isotopes) Noble gases are inert and volatile 4He is an alpha particle, produced principally by a-decay of U and Th, enriching primordial 4He; 3He is largely primordial (constant) The mantle is continually degassing and He lost (cannot recycle back) 4He enrichment expressed as R = (3He/4He) [unusual in that radiogenic is the denominator] Common reference is RA (air) = 1.39 x 10-6 Common assumptions: • Shallow mantle MORB source is relatively homogeneous and depleted in He • Deeper mantle has more primordial (high) 3He/4He, but still degassed and less than primordial (100-200RA) values • PHEM may be that more primitive reservoir • Low 3He/4He may be due to recycled crustal U and Th

  12. Kellogg et al – stealth layer

  13. The stealth layer

  14. Generation of piles by convection

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