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 fO2fO2 from −3.65 to +1.46 relative to the iron–wüstite buffer (IW). Experiments were performed on martian and terrestrial basalts at 1.2 GPa 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 fO2fO2 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 (∼0.5 wt% H2O) conditions, small amounts of CH4 are present. Concentrations of C diminish as conditions become more reduced, amounting to 10 s to ∼100 ppm in the interval ∼IW−2 to IW + 1 where CO species dominate, and as little as 1–3 ppm 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 fO2fO2, 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.