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Geochemical Constraints on Archean Mantle Dynamics

Geochemical Constraints on Archean Mantle Dynamics. S.W. Parman Durham University. Collaborators: Stan Hart, Mark Kurz, Tim Grove, Mike Walter, Graham Pearson, Geoff Nowell, Jeroen van Hunen. Geochemistry and Geodynamics.

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Geochemical Constraints on Archean Mantle Dynamics

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  1. Geochemical Constraints on Archean Mantle Dynamics S.W. Parman Durham University Collaborators: Stan Hart, Mark Kurz, Tim Grove, Mike Walter, Graham Pearson, Geoff Nowell, Jeroen van Hunen

  2. Geochemistry and Geodynamics • Historically has been focused on reservoirs (primitive mantle, EM1, EM2, HIMU,D”…) • New data constrain ages and timescales of mantle processes

  3. Main points • Helium isotopes do not require a primitive mantle or layered mantle convection • Mantle evolution is punctuated by large melting events in the Archean/Proterozoic • The source of plumes (ocean islands) has ancient (> 2 Ga) heterogeneities in it • Mixing time in the convecting mantle is < 2 Ga old

  4. Pattern of Mantle Convection?

  5. Layered mantle convection DHe<<DU+Th Allegre, Kurz, Moreira, Porcelli…

  6. Zindler and Hart, 1986

  7. Graham, 2002 Graham, 2002

  8. Ar constraints Allegre et al., 1996

  9. So what’s the problem?

  10. lower mantle van der Hilst et al., 1997

  11. Graham, 2002 Graham, 2002

  12. Ellam and Stuart, 2004

  13. Layered mantle convection DHe<<DU+Th Allegre, Kurz, Moreira, Porcelli…

  14. If He more compatible than U+Th… DHe>DU+Th

  15. Parman et al., Nature 2005

  16. If He more compatible than U+Th… DHe>DU+Th

  17. Implications • Primitive mantle not required by He isotopes • Whole mantle convection is consistent with data Davies, 2002

  18. He isotopes do not require the existence of primitive mantle

  19. Xie & Tackley, 2004

  20. If He more compatible than U+Th… DHe>DU+Th

  21. Database kindly provided by Atosa Abedini, USGS

  22. Iceland and Hawaii Condie, 1998 Kemp et al., 2006

  23. Parman, Nature, 2007

  24. Davies, EPSL (1995)

  25. Implications • Mantle convection in Archean was punctuated by large melting events • These events appear to have produced large areas of the continental crust Davies, EPSL (1995)

  26. Where is plume source? Davies, 2002

  27. Burke & Torsvik, 2004

  28. Wen, EPSL 2006

  29. plume conv. conv.

  30. Pearson, Parman and Nowell, Nature, 2007

  31. Implications • Plume source is ancient • Convecting mantle (MORB source) is <2 Ga old Ferrachat & Ricard, 1998

  32. The End

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