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The Origin of Precious Metals on Earth
- 1. The Origin of Precious Metals on Earth Tim Elliott Bristol Isotope Group, School of Earth Sciences, University of Bristol
- 2. Precious Metals PGE Highly siderophile elements (HSE)
- 3. Geochemical Reservoirs Bulk Silicate Earth Bulk Earth (Chondritic) Core
- 4. Highly Siderophile Element (HSE) Abundances Walker 2009
- 5. Depletion of Siderophile Elements Drake and Righter 02
- 6. Geochemical Reservoirs Bulk Silicate Earth Bulk Earth (Chondritic) Dmetal-silicate Core Most HSE compatible during mantle melting
- 7. Surprisingly Abundant Precious Metals Drake and Righter 02
- 8. Deep, high temperature core formation? Wood et al 06
- 9. Higher P experiments Mann et al 11
- 10. Late Veneer • Addition of chondritic material post core formation • Requires ~0.5% by mass of Earth by such late meteorite bombardment
- 11. Siderophile element abundances Drake and Righter 02
- 12. NASA image Fessler and Spudis LPI
- 13. Timing of Core Formation Rudge et al 2010
- 14. ‘Late’ Bombardment Image from Elements Magazine
- 15. Late Veneer • Material certainly added post core formation • Need ~20:1 Earth:Moon accretion • Can this amount of material be delivered without being added to core?
- 16. Late Veneer vs High T core • Internal or external source of precious metals • Sample Earth before Late Veneer?
- 17. The oldest part of the planet Jack Hills, Yilgarn Block, W. Australia Wilde et al 01
- 18. Zircon ages Wilde et al 01
- 19. Archaean Greenland Kamber et al 02
- 21. Istaq Gneisses, Greenland Kamber et al 02
- 22. Extinct 146 Sm 146 Sm - 142 Nd (t1/2 ~ 103 67Ma) Sample 142 Nd/144Nd ε142Nd = Ames Nd -1 x10000 142 Nd/144Nd
- 23. Oldest Terrestrial Rocks Isua, Greenland ~3.8Ga ε Caro et al 2006
- 24. Precious Metal Content of Earliest Mantle? • Need to see through melting, crystallisation and metamorphism • Hard to constrain moden mantle Platinum Group Elements (PGE) contents even with samples of mantle • Need a robust proxy for PGE • Isotopic tracer of terrestrial vs extra-terrestrial PGE?
- 25. W proxy of Precious Metals PGE
- 26. Extinct 182 Hf 182 Hf – W (t1/2 ~ 8.9Ma) 182 Sample 182 W/184W ε182W = NIST SRM 3163 -1 x10000 182 W/184W
- 28. Influence of Late Veneer on ε182W
- 29. W isotopic measurements •Multi-collector inductively coupled plasma mass spectrometer (MC-ICPMS), Thermo Finnigan Neptune
- 30. ε182W variability in modern mantle? • Entrainment of core material into mantle via plumes may lower ε182W • High 186Os/188Os of Hawaiian plume attributed to core component (Walker and Brandon) • Outer core should have low ε182W ~-1.9 and much W
- 32. Large Igneous Provinces Coffin 2006
- 33. Deep-seated plumes Montelli et al 2006
- 34. Mantle ε182W variability ‘Deep rooted’ OIB Large Igneous Provinces
- 36. Mantle W isotope evolution ε182W 3.8Ga Greenland (Isua) Modern 2Ga Granitoid
- 37. Influence of Late Veneer on ε182W
- 38. Origin of Precious Metals • Homogeneity of ε182W of modern mantle leaves no room for precious metal deliver from core to mantle • Contrast of 0.15 ε182W between modern and Isua rocks entirely compatible with Late Veneer • Current inventory of precious metals delivered to Earth by meteorite bombardment post core formation • 20x higher flux to Earth compared to Moon a lucky chance of the random nature of accretion (Botke et al 2011)
Editor's Notes
- Confusing 143 referenced to chur (chondrite) as assumed the same, for 142 clearly not the case and for covenience refed to Earth
- Half life on 103Ma so the signature must have been imparted before formation at 3.8
- Confusing 143 referenced to chur (chondrite) as assumed the same, for 142 clearly not the case and for covenience refed to Earth
- Have checked strongest current plume, but is a tail. Worth checking out heads- most head on continents and very prone to contamination, however W. Pacific LIPs oceanic and mark major pulse of volcanism, strongly argued to be from lower mantle. Worth looking at other plumes. Notably deep root of hawaii still not especially clear, whereas as other less vigorous volcanoes aparently have (or did when we started)
- Montelli 2004 banana doughnutCanaries and Azores w resolved, deep rooted origins- (Canaries ‘Robust merge with Canaries?) Cape Verde, clearest topographic and geoid anom (Montelli ‘deep’ plume merging with Canaries/Azores. NB Hawaii and Iceland not so defined