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 (fO 2 )] is different from that of the Earth. Using experiments on a lunar basalt composition, we confirm that carbon dissolves as carbonate at an fO 2 higher than -0.55 relative to the iron wustite oxygen buffer (IW-0.55), whereas at a lower fO 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 fO 2 less than IW-0.55 is associated with a decrease in carbon solubility by a factor of 2. Thus, the fO 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.hydrogen | iron carbonyl | magmatic volatiles | experimental petrology M agmas formed through igneous processes on parent bodies in the early solar system span 12 log units in oxidation state, from 6.3 log units of oxygen fugacity (fO 2 ) below the iron wustite oxygen buffer (IW) to 6 log units above the IW (IW−6.3 to IW+6) (1-4). Over the wide range of basalt fO 2 observed in these planetary bodies, thermodynamic data for the C-O-H gas system indicate that there are significant changes in gas phase speciation (5). These differences should be reflected in how volatiles are dissolved in magmas, but carbon speciation experiments for basaltic systems are limited, particularly at low fO 2 . The developing evidence for volatiles in magmas erupted on the Moon (6, 7) as well as on Earth (3,8), Mars (9-12), and Mercury (13) is a strong incentive to improve our understanding of C-O-H speciation and solubility in planetary basaltic magmas.Previous experimental studies on the speciation of carbon in Fe-free silicate systems under reducing conditions or at fO 2 < IW show carbon is dissolved as methane (14-16). However, under more oxidizing conditions in basaltic melt compositions, carbon is dissolved as carbonate (17)(18)(19), and the transition in speciation from methane to carbonate has long been debated as either occurring at more reducing conditions than IW+1 (20, 21) or at more oxidizing conditions (15). Another study (22) shows that a significant amount of C 4+ (carbonate) can be incorporated at extremely low fO 2 (IW−4.5) if Fe and alkalis are present in the melt. Additional experiments on Fe-bearing basalt are clearly required to resolve the conflicts in existing data. The results are important in understanding the residence time of carbon in the mantle and the rate of carbon degassing during the early evolution of terrestrial planets ...

Volcanic glasses observed on the lunar surface have been interpreted as the products of volatile-rich, fire-fountain eruptions. Revised estimates of the water content of primitive lunar magmas have overturned the notion of a volatile-poor Moon 1-4 , but degassing of water-rich vapour during volcanic eruptions is inconsistent with geochemical and petrological observations 5,6 . Although degassing of carbon is compatible with observations, the amount of indigenous carbon in lunar volcanic materials is not well constrained. Here we present high-precision measurements of indigenous carbon contents in primitive lunar volcanic glasses and melt inclusions. From our measurements, in combination with solubility and degassing model calculations, we suggest that carbon degassed before water in lunar magmas, and that the amount of carbon in the lunar lavas was su cient to trigger fire-fountain eruptions at the lunar surface. We estimate-after correcting for bubble formation in the melt inclusions-that the primitive carbon contents and hydrogen/carbon ratios of lunar magmas fall within the range found in melts from Earth's depleted upper mantle 7 . Our findings are also consistent with measurements of hydrogen, fluorine, sulphur and chlorine contents, as well as carbon and hydrogen isotopes, in primitive lunar magmas 2-4,8,9 , suggesting a common origin for the volatile elements in the interiors of the Earth and Moon.Volatile elements, especially C and H, play an important role in the evolution and differentiation of planetary bodies and provide significant constraints on models for the formation of the terrestrial planets 10,11 . The volatile budget of the lunar mantle can, at present, be reconstructed only from the record preserved in the mare basalts and lunar volcanic glasses, the most primitive magmas found on the Moon 9 . Reconstructing the primitive indigenous volatile content of the lunar mantle from basaltic melts requires careful evaluation and correction for volatile degassing during magma upwelling, eruption and contamination from external sources 8 .There has been significant debate regarding the gas species that propelled the lunar magmas and created the fire-fountain eruptions that produced the lunar volcanic glass deposits 12 . Until recently the Moon was thought to be dry, and carbon was proposed as the volatile element responsible for the fire-fountain style of eruptions 13 . Oxidation of as little as 50 ppm graphite could produce the CO-rich gas phase needed to produced a fire-fountain eruption [14][15][16] . Yet, the detection of measurable indigenous carbon concentrations dissolved in the lunar volcanic glasses remained controversial. In situ measurements of indigenous H, S, Cl and F in the lunar volcanic glasses and olivine-hosted melt inclusions indicate that these volatiles might have contributed to the gas phase 1-4 , but they are unlikely to have created the initial gas phase 14 . Two main factors limited the study of carbon abundances in lunar samples: first, bulk sample analyses make it difficult t...