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Degassing of reduced carbon from planetary basalts

Wetzel, Diane T., Rutherford, Malcolm J., Jacobsen, Steven D., Hauri, Erik H., Saal, Alberto E.
Proceedings of the National Academy of Sciences of the United States of America 2013 v.110 no.20 pp. 8010-8013
basalt, carbon, carbon dioxide, evolution, greenhouses, melting, mercury, methane, oxidation, oxygen, solubility, volcanic activity
Degassing of planetary interiors through surface volcanism plays an important role in the evolution of planetary bodies and atmospheres. On Earth, carbon dioxide and water are the primary volatile species in magmas. However, little is known about the speciation and degassing of carbon in magmas formed on other planets (i.e., Moon, Mars, Mercury), where the mantle oxidation state [oxygen fugacity (f O ₂)] is different from that of the Earth. Using experiments on a lunar basalt composition, we confirm that carbon dissolves as carbonate at an f O ₂ higher than -0.55 relative to the iron wustite oxygen buffer (IW-0.55), whereas at a lower f O ₂, we discover that carbon is present mainly as iron pentacarbonyl and in smaller amounts as methane in the melt. The transition of carbon speciation in mantle-derived melts at f O ₂ less than IW-0.55 is associated with a decrease in carbon solubility by a factor of 2. Thus, the f O ₂ controls carbon speciation and solubility in mantle-derived melts even more than previous data indicate, and the degassing of reduced carbon from Fe-rich basalts on planetary bodies would produce methane-bearing, CO-rich early atmospheres with a strong greenhouse potential.