Solubility of CH4 in a synthetic basaltic melt, with applications to atmosphere-magma ocean-core partitioning of volatiles and to the evolution of the Martian atmosphere

P. Ardia, M. M. Hirschmann, A. C. Withers, B. D. Stanley

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66 Scopus citations

Abstract

We employ a double capsule technique to determine the solubility of CH4 in haplobasaltic (Fe-free) liquid under conditions of constrained methane fugacity, fCH4, at pressures of 0.7-3GPa at 1400-1450°C. Dissolved C-O-H species are examined with FTIR and Raman spectroscopy, and CH4 and CH3- are the only C-bearing species detected. Carbon solubilities are quantified using SIMS, range from 70 to 480ppm when calculated as CH4, and increase with pressure. Concentrations are parameterized with a thermodynamic model and are found to be related to fCH4 and pressure. Application of this thermodynamic model shows dissolved CH4 contents of graphite-saturated magmas are little-influenced by pressure for conditions of fixed fO2 relative to metal-oxide buffers and fixed total H content. Because fCH4 of graphite-saturated systems increases with the square of hydrogen fugacity, dissolved fCH4 increases with decreasing fO2 and increases exponentially with increasing total H content. The experimentally-observed increase with pressure is related to variations in fO2 and H content. Dissolved CH4 contents of Martian magmas in their source regions are small, such that it is unlikely that magmatic CH4 is a principal contributor to greenhouse conditions early in Martian history. Concentrations of dissolved C-O-H volatiles in a magma ocean early in the history of a terrestrial planet may be diminished by reducing conditions, leading to development of a massive atmosphere and a greatly decreased inventory of volatiles stored in planetary interiors at the outset of planetary history. Dissolution of methane may enhance the retention of C in the silicate Earth during core formation, but experimental evaluation of its influence on metal/silicate partitioning of C requires careful matching of the magmatic H concentration between experiments and natural systems.

Original languageEnglish (US)
Pages (from-to)52-71
Number of pages20
JournalGeochimica et Cosmochimica Acta
Volume114
DOIs
StatePublished - Aug 1 2013

Bibliographical note

Funding Information:
We appreciate conversations with Bjorn Mysen early in the history of this study and the comments of referees Fabrice Gaillard, Francis McCubbin, and associate editor Michael Toplis. We gratefully acknowledge support from NASA (NNX11AG46G and NNX11AH13G) and from NSF (EAR0757903 and EAR1161023).

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