Geochemical constraints on mantle structure and convection

1. Geochemical constraints on mantle structure and convection

4. Geochemical constraints on mantle flow • Heat flow budget. 44TW coming out of Earth but 6TW generated by radioactivity in continental crust. Maybe 10TW come from core, 20TW from radioactivity, leaving about 8TW from cooling. • BUT only 4TW generated by radioactivity in MORB source. If this were true of whole mantle, would need ridiculous cooling rate to get observed heat flux • Noble gases (Ar and He in particular) argue for an undegassed part of mantle • Basalts (OIB and MORB) -- imply a range of distinct reservoirs --recycling of oceanic crust and lithosphere -- about 50 percent of mantle has been depleted -- implies a relatively undepleted reservoir • Where do we put the primitive reservoir?

5. The 40Ar budget

6. 40Ar budget continued • Can estimate Uranium content for the mantle from cosmochemical arguments – then estimate potassium content from usual ratios. • This gives about 20TW in radiogenic heat production (only 10% of this is from 40K) • About 40% of expected 40Ar is in the atmosphere and about 10% in the CC. Leaves 50% undegassed. • Numerical degassing models (without hidden deep reservoirs) which consider only degassing at ridges (heat is lost everywhere) do give about 50% of the 40Ar in the atmosphere • So Argon budget is not a strong argument for a hidden reservoir

7. Heat and 4He imbalance • Decay of 238U, 235U, and 232Th produces both heat and 4He in constant proportions. Can estimate both from radiogenic heat budget. • 3He is primordial and exists in the Earth (so Earth never totally degassed) • Use R=3He/4He as measure of enrichment with a common reference being Ra for air • Helium is constantly being degassed – too light to be recycled • Find about an order of magnitude less 4He being degassed than expected • Numerical models using degassing zones at ridges give very time-dependent He/Heat fluxes – but rarely gets as low as observed.

10. (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

11. 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.

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

13. OIBs classified by different isotopic signatures (clusters on isotope plots): • HIMU Tubuai, Austral Islands, Azores, Balleny Islands • EM1 Tristan, Pitcairn, Inaccessible Island • EM2 Societies, Samoa hotspots

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

15. Apparent age is 2 Gyr No simple trend -- CC is in middle -- therefore not back-mixing of CC into mantle Given incompatibilities, MORB should fall to left of geochron, CC to right Lead paradox – hydrothermal processes lead to lead being transferred to CC

16. Note convergence of HIMU and MORB at FOZO

17. 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

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

20. (Other) sources of mantle compositional heterogeneity:

23. Kellogg et al – stealth layer

24. The stealth layer

25. Generation of piles by convection