Partitioning of Carbon Between Fe-Ni Alloy Melt and Silicate Melt at Graphite Saturation- Implications for the Budget and Origin of Volatiles in Earth, Mars, and the Moon
Master of Science
The budget and origin of carbon in Earth and other terrestrial planets are debated and one of the key unknowns is the behavior and fate of carbon during early planetary processes including accretion, core formation, and magma ocean crystallization. Here we determine, experimentally, the solubility of carbon in coexisting Fe-Ni alloy melt and basaltic silicate melt in shallow magma ocean 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 from IW-0.37 and IW-1.02, 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, with silicate melt NBO/T (estimated proportion of non-bridging oxygen with respect to tetrahedral cations) 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 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. 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 (non-bridging per tetrahydron, an index of the depolymerization of the silicate melt), the oxygen fugacity and the water content of the silicate melts. Raman and FT-IR results show that at our experimental conditions, carbon in silicate melt is dissolved as hydrogenated species, in addition to . The calculated carbon partition coefficient varies from 510±53 to 5369±217 and varies systematically as a function of P, T, fO2, water content and the composition of the silicate melt. The range of measured in our study with carbonated and hydrogenated carbon species in silicate melt is similar to that reported in literature for experiments where carbonyl complexes are chief carbon species in silicate melts. An empirical parameterization was derived using the data from this and existing studies such as LnDc = a/T + b* P/T + c * ln(fO2) + d*(NBO/T) + e; where a = -16934, b = 1074, c = -0.403, d = -1.229, e = 8.611,the temperature is in Kelvins, and the pressure is in GigaPascals. Using this parameterization and the estimated conditions for the base of the magma oceans, we estimate the average value for Earth, Mars, and the Moon. The deepest magma ocean of Earth results in the strongest depletion of its silicate carbon budget, followed by Mars with intermediate depth magma ocean, and then the Moon with a shallow magma ocean. We predict that the lunar mantle carbon budget similar to that of the Earth’s upper mantle might have been set by equilibrium core-mantle fractionation in magma ocean, whereas for Earth, later processes such as ingassing from proto-atmosphere and late-stage accretion of volatile-rich material was necessary for delivery of carbon and other volatiles.
Carbon partitioning; Silicate melts; Fe-Ni Alloy; Graphite saturation