Ben D. Stanley, Marc M. Hirschmann and Anthony C. Withers

Department of Earth Sciences, University of Minnesota, Minneapolis, MN, 55455, USA

To determine the speciation and concentrations of dissolved C-O-H volatiles in graphite-saturated martian primitive magmas, we conducted piston-cylinder experiments on graphite-encapsulated synthetic melt of Adirondack-class Humphrey basaltic composition. Experiments were performed over three orders of magnitude in oxygen fugacity (IW+2.3 to IW-0.8), and at pressures (1-3.2 GPa) and temperatures (1340-1617 °C) similar to those of possible martian source regions. Oxygen fugacities were determined from compositions of coexisting silicate melt+FePt alloy, olivine+pyroxene+FePt alloy, or melt+Fe-C liquid. Infrared spectra of quenched glasses all show carbonate absorptions at 1430 and 1520 cm^-1, with CO₂ concentrations diminishing under more reduced conditions, from 0.50 wt% down to 26 ppm. Carbon contents of silicate glasses and Fe-C liquids were measured using secondary ion mass spectrometry (SIMS) yielding 36-716 ppm and 6.71-7.03 wt%, respectively. Fourier transform infrared (FTIR) and SIMS analysis produced similar H₂O contents of 0.26-0.85 and 0.29-0.40 wt%, respectively. Raman spectra of glasses reveal evidence for OH^– ions, but no indication of methane-related species. FTIR-measured concentrations of dissolved carbonate diminish linearly with oxygen fugacity, but more reduced conditions yield greater dissolved carbonate concentrations than would be expected based on oxidized conditions in previous work. C contents of silicate glasses determined by SIMS are consistently higher than C as carbonate determined by FTIR. Their difference, termed non-carbonate C, correlates well with additional IR absorptions found in reduced glasses (f_O2<IW+0.4) at 1615, 2205, and 3370 cm^-1. These absorption bands are not seen in more oxidized glasses, except B441 (IW+1.7), presumably because they represent reduced C-bearing complexes. The 2205 cm^-1 peak is attributed to a C=O complex, possibly an Fe-carbonyl ion. The 1615 cm^-1 peak does not correlate with that at 2205 cm^-1, but does correlate with non-carbonate C and is in a region commonly associated with C=O bonding. The origin of the peak at 3370 cm^-1 is poorly understood and could potentially be owing to a variety of C-O-H species or to N-H bonding. The intensities of the 1615 and 3370 cm^-1 peaks correlate with each other leading us to provisionally attribute both to an unspecified complex with both C=O and N-H bonds. These results suggest that dissolved species such as carbonyl or other C=O-bearing species could be a significant source of C fluxes to the martian atmosphere, with minor additions of CO₂ and negligible methane contributions. By assuming that degassed, reduced C ultimately becomes atmospheric CO₂, reduced C outgassing may be incorporated in models of martian atmospheric evolution. At Humphrey source region conditions (1350±50 °C, 1.2±0.1 GPa) the total C contents are equivalent to 1200 ppm CO₂ at IW+1 and 475 ppm CO₂ at IW, which are 2 and 4 times higher than the CO₂ derived from CO₃^2- alone. For reasonable magmatic fluxes over the last 4.5 Ga of martian history, such graphite-saturated magmas would produce 0.25 and 0.60 bars from sources at IW and IW+1, significantly more than expected solely from consideration of dissolved CO₂. The carbon contents of Fe-C liquids in this study are consistent with graphite-saturated carbide liquids becoming more C-rich with increasing temperature. Experiments with melt and Fe-C liquid have values of  between 1.3×10³ and 2.2×10³, potentially allowing planetary mantles to retain significant C following core formation.

Reference
Stanley BD, Hirschmann MM and Withers AC (in press) Solubility of C-O-H volatiles in graphite-saturated martian basalts. Geochimica et Cosmochimica Acta
[doi:10.1016/j.gca.2013.12.013]
Copyright Elsevier

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