Earth’s Deep Carbon Cycle

1. Earth’s Deep Carbon Cycle Marc Hirschmann University of Minnesota COMPRES, June, 2006 Thanks to: + Neil Smith, Nikki Dellas (undergrad researchers) Raj Dasgupta

2. 88 ppm 1400 ppm H2O Concentration in Mantle

4. The Carbon Cycle Long-Term Carbon Cycle

5. Carbon Fluxin & out of the Mantle (Alt & Teagle, 1999; Sleep & Zahnle, 2001; Jarrard, 2003) (Total surface carbon=7 X 1022 g)

6. Flux of Subducted Carbon (Kerrick & Connolly, 2001; Jarrard, 2003)

7. Flux of Subducted Carbon (Molina & Poli, 2000) Carbonates remain as refractory phase in the residue as crust dehydrates (Yaxley & Green, 1994; Kerrick & Connolly, 2001; Connolly, 2005; Goran et al., 2006)

8. Release of Subducted Carbon – how, where? Partial melting of carbonated eclogite (eclogite + CO2) is likely to control the depth of release of crustal carbon in the mantle

9. Solidus Detection – Carbonated Eclogite Subsolidus Supersolidus Dasgupta et al. (2004) - EPSL

10. Solidus of Carbonated Eclogite (SLEC1) Dasgupta et al. (2004) - EPSL

11. Deep Cycling of Carbon (Yasuda et al., 1994; Hirschmann, 2000; van Keken et al., 2002; Peacock et al., 2003) Dasgupta et al. (2004) - EPSL

12. Fate of Carbonated Eclogite in Upwelling Mantle Deep Melting of Carbonated Eclogite May account for seismic low-velocity zone atop the 410 km discontinuity (e.g., Song et al., 2004) Dasgupta et al. (2004) - EPSL

13. If carbonated eclogite melts very deep, solidus of peridotite + CO2 controls loci of Carbon storage and release

14. Solidus of Carbonated Peridotite (PERC) Dasgupta & Hirschmann (2006) - Nature

15. Peridotite Solidus – H2O versus CO2 Dasgupta & Hirschmann (in revision) - AmMin

16. Solidus of Mantle Peridotite with trace CO2 Dasgupta & Hirschmann (2006) - Nature

17. Seismic Evidence of Deep Melting ? MELT Experiment beneath EPR Hammond&Toomey (2003) Gu et al. (2005)

18. Deep Melting Beneath Mid-Ocean Ridges Dasgupta & Hirschmann (2006) - Nature

19. Deep Melting of Peridotite + CO2 – implication for mantle geochemistry Dasgupta & Hirschmann (2006) - Nature Residue Depletion Increasing Compatibility

20. Fraction of Mantle Undergoing Deep Melting 3  1018 g of mantle/ yr 75% of the mantle/ Ga 25% of the mantle/ Ga 1  1018 g of mantle/ yr Dasgupta & Hirschmann (2006) - Nature

21. Solidus of Carbonated Peridotite (PERC) Dasgupta & Hirschmann (2006) - Nature

24. Trace amounts of H2O enhance melting…. Hirth & Kohlstedt, 1996; Asimow & Langmuir, 2003

25. Origin of Chemical Lithosphere and Volatile induced Melting of Peridotite Hirth & Kohlstedt (1996) Aubaud et al., 2004 Peridotite Dehydration is aided by Partial Melting

26. Deep Carbonated Silicate Melting Beneath Ridges Transition from carbonatite to carbonated silicate melts at 3 GPa Silicate Melting is enhanced in presence of carbonates Dasgupta & Hirschmann (submitted)

28. Deep Carbonated Silicate Melting & H2O Extraction Beneath Ridges

31. Oxygen fugacity of Transition Zone Is likely to be highly reducing McCammon, Science, 2005

32. Redox Melting: C + O2 + Mg2SiO4 -> MgCO3 (melt) + MgSiO3

33. Channels on Venus Up to 7000 km long! Most assume these are lava channels Basalts, komatiites or carbonatites?

34. Conclusions The biosphere exists by permission of the mantle Carbonated silicate melting is likely in many or all loci hydrous silicate melting in the mantle, with possibly key effects The distribution of oxidized and reduced carbon in the mantle remains uncertain.

35. Ferric/ferrous iron in MORB glasses Bezos & Humler, 2005

36. Effect of Variable CO2 on the Solidus of Mantle Peridotite Solidus of Mantle Peridotite with 120-1200 ppm CO2 is likely to be ~50 °C less than PERC Dasgupta & Hirschmann (2006) - Nature

37. Influence of Na on Carbonated Peridotite Solidus