HR: 1340h
AN: DI33A-1121    [Abstracts]
TI: Carbon Solubility in Core Melts in Shallow Magma Ocean Environment and its bearing on Distribution of Carbon between Deep Earth Reservoirs
AU: * Dasgupta, R
EM: rajdeep@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, United States
AU: Walker, D
EM: dwalker@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, United States
AB: Carbon affects the melting phase relations of mantle rocks [1] and core metal [2], influences the physical properties of molten silicates and metals, and also has significant effect on partitioning of other key elements between various deep Earth phases. But the carbon budget of Earth's deep mantle and core is poorly constrained due to lack of knowledge of behavior of carbon during core formation. In order to determine the storage capacity of dissolved carbon in metallic core melts and to put constraints on partitioning of carbon between silicate mantle and metallic core, we have determined the solubility of carbon in molten core metal at P- T conditions relevant for a shallow magma ocean.
Experiments are performed at 2 GPa and to 2500 °C using a piston cylinder apparatus. Pure Fe-rod or a mixture of Fe-5.2%Ni loaded into graphite capsules were used as starting materials. Al coated run products are analyzed by EMP. Carbon concentration of 5.8 ± 0.4 wt.% at 2000 °C, 6.5 ± 0.9 wt.% at 2250 °C, and 7.5 ± 1.2 wt.% at 2500 °C are measured in quenched iron melt saturated with graphite. The trend of C solubility versus temperature for Fe-5.2 wt.% Ni melt, within analytical uncertainties, is similar to that of pure Fe.
We have compared our solubility data and an estimate of the current carbon content of the mantle with the carbon content of core melts and residual mantle silicates respectively, derived from equilibrium batch or fractional segregation of core liquids, to constrain the partition coefficient of carbon between silicate and metallic melts in a magma ocean, D_C. Translation of the limits of D_C, derived from our solubility data, on calculation of carbon content of the residual silicate shows that the observed mantle concentration of carbon is too low to be matched by the process of shallow magma ocean fractionation of carbon between metal and silicate in a chondritic protoearth. If carbon solubility in liquid Fe does not change strongly as a function of pressure, this may indicate the presence of a hidden carbon- rich mantle reservoir untapped by oceanic volcanism. For the entire range of possible bulk Earth carbon content from chondritic to subchondritic and for the mantle carbon content of 50-1000 ppm, D_C of 10^-4- 10⁰ are derived. But for 1000 ppm bulk Earth carbon, D_C is 10^-2-10⁰. Using the complete range of possible D_C for a magma ocean at ~2200 °C, we predict maximum carbon content of the Earth's core to be 6-7 wt.% and a preferred value of 0.25 ± 0.15 wt.% carbon for a bulk Earth carbon concentration of 1000 ppm. Based on our estimate, the core is likely one of the most enriched terrestrial reservoirs of carbon with concentration as high as 0.4 wt.%, which likely is at least an order of magnitude higher than that of the average mantle. The higher carbon content of OIBs compared to MORBs thus may derive in part from core contributions to mantle plumes.
[1] Dasgupta, R. and Hirschmann, M.M. 2006, Nature 440, 659- 662. [2] Wood, B.J. 1993, Earth Planet Sci Lett 117, 593-607.
DE: 1015 Composition of the core
DE: 1025 Composition of the mantle
DE: 1038 Mantle processes (3621)
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3630 Experimental mineralogy and petrology
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting