Speciation and solubility of reduced C-O-H-N volatiles in mafic melt: Implications for volcanism, atmospheric evolution, and deep volatile cycles in the terrestrial planets

Using vibrational spectroscopy and SIMS, we determined the solubility and speciation of C-O-H-N dissolved volatiles in mafic glasses quenched from high pressure under reduced conditions, with fO2 from -3.65 to +1.46 relative to the iron-wüstite buffer (IW). Experiments were performed on martian and terrestrial basalts at 1.2GPa and 1400°C in graphite containers with variable availability of H₂O, and in the presence of FePt alloys or Fe-C liquids. The dominant C-O-H-N species varies systematically with fO2 and H₂O content: the carbonate ion prevails above IW+1, but for dry conditions between IW-2 and IW+1, CO species are most important. Below IW, reduced NH-bearing species are present. At the most reducing and hydrous (~0.5wt% H₂O) conditions, small amounts of CH₄ are present. Concentrations of C diminish as conditions become more reduced, amounting to 10s to ~100ppm in the interval ~IW-2 to IW+1 where CO species dominate, and as little as 1-3ppm at more reduced conditions. Concentrations of non-carbonate carbon, dominated by CO species, correlate with CO fugacities along a trend implying that the species stoichiometry has just one CO group and suggesting that carbonyl complexes (transition metals with multiple carbon monoxide ligands) are not important species under these conditions. C partition coefficients between Fe-C liquid and silicate melt increase with decreasing fO2, becoming as great as 10⁴ for the most reducing conditions investigated. The low solubility of C in silicate liquids under reducing conditions means that most C during the magma ocean stage of planetary differentiation is either segregated to the core or in the overlying atmosphere. Precipitation of C-rich phases in a carbon-saturated magma ocean is also possible, and is one mechanism by which some C can be retained in the mantle of a planet. The predominant magmatic carbonaceous species for both martian and lunar volcanism is likely CO.