Degassing of lunar basalts and the composition of the Bulk Silicate Moon
IPGP - Îlot Cuvier
Degassing of planetary interiors through surface volcanism plays an important role in the evolution of planetary bodies and atmospheres. The effect of degassing processes on the volatile and moderately volatile elements of erupted lavas has to be evaluated before we infer the bulk composition of any planetary body. Little is known about the speciation and degassing of carbon in magmas formed on other planets where the oxygen fugacity is different from that of the Earth. Carbon dissolves as carbonate at a fO2 higher than -0.55 relative to the iron wustite oxygen buffer (IW-0.55), whereas at a lower fO2, 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 fO2 less than IW-0.55 is associated with a decrease in carbon solubility by a factor of 2. We present the first high-precision, in-situ indigenous carbon contents in primitive lunar volcanic glasses and melt inclusions, reaching up to 4 ppm. We used recent C-O-H solubility data to determine the CO-H2O saturation pressure and the composition of the equilibrium gas phase. Our study indicates that carbon mostly degasses before water. Water is primarily lost post-melt fragmentation by kinetic degassing, suggesting that the highest water content and lowest D/H measured in the lunar melt inclusions represent primitive lunar values. Surprisingly, once corrected for bubble formation, the H/C ratios of the melt inclusions are similar to those in Earth’s depleted upper mantle melts. The estimated “Bulk Silicate Moon” composition for highly volatile elements, constrained by volatile/refractory element ratios in the melt inclusions, is only moderately depleted (avg. 0.20 BSE) compared with the Bulk Silicate Earth.