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

Lora S. Armstrong*, Marc M. Hirschmann, Ben D. Stanley, Emily G. Falksen, Steven D. Jacobsen

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

41 Scopus citations

Abstract

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.

Original languageEnglish (US)
Pages (from-to)283-302
Number of pages20
JournalGeochimica et Cosmochimica Acta
Volume171
DOIs
StatePublished - Dec 15 2015

ASJC Scopus subject areas

  • Geochemistry and Petrology

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