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    CO2 Solubility in Primitive Martian Basalts Similar to Yamato 980459 and the Evolution of the Martian Atmosphere
    (2012-12-12) Stanley, Ben D.; Schaub, Douglas R.; Hirschmann, Marc M.
    There is considerable evidence that liquid water was stable on the ancient Martian surface during at least some parts of the late Noachian and early Hesperian epochs. Yet there remains considerable uncertainty as to how this greenhouse was created and maintained and how it evolved to the current thin, modern atmosphere. To understand possible volcanogenic fluxes of CO2 to the early Martian atmosphere, we investigated experimentally carbonate solubility in a synthetic melt based on the shergottite meteorite Yamato 980459 (Y980459), a picritic rock (19.08 wt.% MgO) thought to be a near-primary liquid derived from high temperature (>1540 °C) partial melting of the Martian mantle at high temperature. Previous work on the CO2 solubility in synthetic Martian basalts based on the Humphrey Adirondack-class basalt showed that degassing of CO2 to the atmosphere may not be sufficient to create greenhouse conditions required by observations of liquid surface water (Stanley et al, 2011, GCA). However, solubilities are predicted to be greater for depolymerized melts similar to Y 980459, possibly allowing degassing of increased amounts of dissolved CO2 and a significant contribution of volcanogenic CO2 to an early Martian greenhouse. A series of piston-cylinder experiments were performed at 1-2 GPa and 1600-1650 ºC in Fe-doped Pt capsules. CO2 contents in experimental glasses were determined using Fourier transform infrared spectroscopy (FTIR) and range from 0.45-1.26 wt.%. These are nearly identical to solubilities documented for Humphrey basalt and do not show enhanced solubility owing to depolymerization. Thus the experimentally-determined solubility of synthetic picritic shergottite basalts confirm that the Martian mantle is incapable of degassing sufficient CO2 to sustain a thick greenhouse atmosphere in the Late Noachian. Therefore, models of Martian atmospheric evolution considering only the greenhouse effects of CO2 should be reexamined and additional volatiles such as SO2 and CH4 should be considered.
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    Solubility of C-O-H volatiles in graphite-saturated martian basalts and application to martian atmospheric evolution
    (2012-12-12) Stanley, Ben D.; Hirschmann, Marc M.; Withers, Anthony C.
    The modern martian atmosphere is thin, leading to surface conditions too cold to support liquid water. Yet, there is evidence of liquid surface water early in martian history that is commonly thought to require a thick CO2 atmosphere. Our previous work follows the analysis developed by Holloway and co-workers (Holloway et al. 1992; Holloway 1998), which predicts a linear relationship between CO2 and oxygen fugacity (fO2) in graphite-saturated silicate melts. At low oxygen fugacity, the solubility of CO2 in silicate melts is therefore very low. Such low calculated solubilities under reducing conditions lead to small fluxes of CO2 associated with martian magmatism, and therefore production of a thick volcanogenic CO2 atmosphere could require a prohibitively large volume of mantle-derived magma. The key assumption in these previous calculations is that the carbonate ion is the chief soluble C-O-H species. The results of the calculations would not be affected appreciably if molecular CO2, rather than carbonate ion, were an important species, but could be entirely different if there were other appreciable C-species such as CO, carbonyl (C=O) complexes, carbide (Si-C), or CH4. Clearly, graphite-saturated experiments are required to explore how much volcanogenic C may be degassed by reduced martian lavas. A series of piston-cylinder experiments were performed on synthetic martian starting materials over a range of oxygen fugacities (IW+2.3-IW-0.9), and at pressures of 1-3 GPa and temperatures of 1340-1600 ºC in Pt-graphite double capsules. CO2 contents in experimental glasses were determined using Fourier transform infrared spectroscopy (FTIR) and range from 0.0026-0.50 wt%. CO2 solubilities change by one order of magnitude with an order of magnitude change in oxygen fugacity, as predicted by previous work. Secondary ion mass spectrometry (SIMS) determinations of C contents in glasses range from 0.0131-0.2626 wt%. C contents determined by SIMS are consistently higher than CO2 contents determined by FTIR. This difference, termed excess C, is attributed to the presence of other reduced C-species, such as carbonyls and amides (which have C=O and N-H bonds), detected using FTIR in reduced graphite-saturated martian basalts. An atmosphere produced by degassing of magmas similar to this study would be richer in C-O-H species than previously modeled using only CO2 and could create a much warmer climate that stabilizes liquid water on the ancient martian surface.