Stability of reduced carbon in the mantle

Abstract

Reduced carbon in the mantle is commonly thought to be chiefly in diamond, but experiments suggest that at >250 km the mantle contains small amounts (0.1-1 %) of FeNi alloy. [1, 2, 3]. Thus, alloy may be a significant host of reduced C [4], but little is known about C solubility of FeNi alloy under mantle conditions. To determine the carbon solubility in FeNi alloy and melt, we conducted experiments in the system Fe-Ni-C with bulk compositions having 5 wt. % C and variable Fe/(Fe+Ni) at 3 to 7 GPa and 1000 - 1400°C. Experiments at 3 GPa and 1000-1250 °C were performed in an end loaded piston cylinder apparatus; those at 5 and 7 GPa and 1200-1400°C were performed in a 1000 ton Walker-style octahedral multianvil. At 3 GPa, Fe-rich melts contain up to 4.5 wt. % C, but Ni-rich (>25 mole% Ni) compositions remain subsolidus at 1250°C. The solubility of carbon in pure Fe and Ni metal are 2 wt. % and 1 wt. % respectively, but in the alloy passes through a minimum of 0.4 wt. % for Fe0.2Ni0.8. Assuming that these concentrations apply at higher temperatures and pressures (as will be tested by future experiments) allows a first estimate of the potential storage of C in FeNi alloy in the mantle. If the mantle at 250 km contains 0.1-0.2 wt.% Ni-rich (Fe0.4Ni0.6) alloy, increasing with depth to 1 wt.% Fe-rich (Fe0.88Ni0.12) alloy at 700 km [1,2], then maximum storage of C in alloy rises from 5 ppm in the deep upper mantle to 180 ppm in the shallow lower mantle. For mantle similar to the MORB source, with ~10-30 ppm C [5], alloy cannot store all C in the deep upper mantle but can in the lower mantle. For OIB sources with 33-500 ppm C [5], complete storage in alloy is less likely. Additional phases will be diamond in the upper mantle, as our experiments and previous work [6] indicate that carbide is not stable in equilibrium with Ni-rich alloy, and carbide melt in the lower mantle. [1] Frost et al (2004) Nature 428 409-412 [2] Frost and McCammon (2008) EPSL 36 389-420 [3] Rohrbach et al. (2011) J.Petrol 52 #717-731 [4] Dasgupta et al. (2009) GCA 73 6678-6691 [5] Dasgupta and Hirschmann (2010) EPSL 298 1-13 [6] Romig and Goldstein (1978) Metal Trans. Met. AIME 9a 1599-1609.