Han Chi^a, Rajdeep Dasgupta^a, Megan Duncan^a and Nobumichi Shimizu^b

^aDepartment of Earth Science, Rice University, 6100 Main Street, MS 126, Houston, TX 77079, USA
^bDepartment of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

The budget and origin of carbon in Earth and other terrestrial planets are debated and one of the key unknowns is the fate of carbon during early planetary processes including accretion, core formation, and magma ocean (MO) crystallization. Here we determine, experimentally, the solubility of carbon in coexisting Fe-Ni alloy melt and basaltic silicate melt in shallow MO conditions, i.e., at 1-3 GPa, 1500-1800 °C. Oxygen fugacity of the experiments, estimated based on Fe (in metallic alloy melt)-FeO (in silicate melt) equilibrium, varied between ∼IW-0.4 and IW-1.0, where IW refers to the oxygen fugacity imposed by the coexistence of iron and wüstite. Four different starting mixes, each with 7:3 silicate:metal mass ratio and silicate melt NBO/T (estimated proportion of non-bridging oxygen with respect to tetrahedral cations; , where T = Si + Ti + Al + Cr + P) ranging from 0.81 to 1.54 were studied. Concentrations of carbon in the alloy melt were determined using electron microprobe whereas carbon contents of quenched basaltic glasses were determined using secondary ionization mass spectrometry (SIMS). Identification of carbon and hydrogen-bearing species in silicate glasses was performed using Raman and Fourier Transformed Infrared (FTIR) spectroscopy.

Our results show that carbon in the metallic melt varies between 4.39 and 7.43 wt.% and increases with increasing temperature and modestly with increasing pressure but decreases with increasing Ni content of the alloy melt. Carbon concentration in the silicate melts, on the other hand, varies from 11±1 ppm to 111±7 ppm and is negatively correlated with pressure but positively correlated with temperature, the NBO/T, the oxygen fugacity and the water content of the silicate melts. Raman and FTIR results show that at our experimental conditions, carbon in silicate melt is dissolved both as hydrogenated species and . The calculated carbon partition coefficient varies from 510±53 to 5369±217 and varies systematically as a function of P  , T  , f_O2, water content, the composition of the silicate melt (expressed using NBO/T), and Ni content of alloy melt (X  _Ni). The range of  measured in our study with carbonated and hydrogenated carbon species in silicate melt is similar to that reported in the literature for experiments where carbonyl complexes are the chief carbon species in silicate melts. An empirical parameterization was derived using the data from this and existing studies such as

where a   = -33510, b   = 1357, c   = -0.596, d   = -1.18, e   = 4.15, f   = 13.37,the temperature is in Kelvins, and the pressure is in gigapascals. Using this parameterization and the estimated conditions for the base of the MOs, the average  value for Earth, Mars, and the Moon can be predicted. The deep MO of Earth is predicted to cause the strongest depletion of its silicate carbon budget, closely followed by Mars with intermediate depth MO, and then the Moon with a shallow MO. We predict that the lunar mantle carbon budget, similar to that of the Earth’s present-day upper mantle, might have been set by equilibrium core-mantle fractionation in MO; whereas for Earth, later processes such as ingassing from a proto-atmosphere and late-stage accretion of volatile-rich material was necessary for delivery of carbon and other volatiles. Finally, the comparison of our measured and predicted value of for terrestrial MO with similar constraints on from the literature suggests that the apparent depletion of nitrogen relative to carbon for the bulk silicate Earth and the Earth’s upper mantle is unlikely to be caused by preferential partitioning of nitrogen to alloy melt during core formation.

Reference
Han Chi H, Dasgupta R, Duncan M and Shimizu N (in press) Partitioning of Carbon between Fe-rich Alloy Melt and Silicate Melt in a Magma Ocean – Implications for the Abundance and Origin of Volatiles in Earth, Mars, and the Moon. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.04.046]
Copyright Elsevier

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Harju ER, Rubin AE, Ahn I, Choi B-G, Ziegler K and Wasson JT (in press) Progressive aqueous alteration of CR carbonaceous chondrites. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2014.04.048]
Copyright Elsevier

Link to Article