Solubility of water in lunar basalt at low pH2O

Abstract

We report the solubility of water in Apollo 15 basaltic “Yellow Glass” and an iron-free basaltic analog composition at 1atm and 1350°C. We equilibrated melts in a 1-atm furnace with flowing H2/CO2 gas mixtures that spanned ∼8 orders of magnitude in fO2 (from three orders of magnitude more reducing than the iron-wüstite buffer, IW−3.0, to IW+4.8) and ∼4 orders of magnitude in pH2/pH2O (from 0.003 to 24). Based on Fourier transform infrared spectroscopy (FTIR), our quenched experimental glasses contain 69–425ppm total water (by weight). Our results demonstrate that under the conditions of our experiments: (1) hydroxyl is the only H-bearing species detected by FTIR; (2) the solubility of water is proportional to the square root of pH2O in the furnace atmosphere and is independent of fO2 and pH2/pH2O; (3) the solubility of water is very similar in both melt compositions; (4) the concentration of H2 in our iron-free experiments is <∼4ppm, even at oxygen fugacities as low as IW−2.3 and pH2/pH2O as high as 11; (5) Secondary ion mass spectrometry (SIMS) analyses of water in iron-rich glasses equilibrated under variable fO2 conditions may be strongly influenced by matrix effects, even when the concentration of water in the glasses is low; and (6) Our results can be used to constrain the entrapment pressure of lunar melt inclusions and the partial pressures of water and molecular hydrogen in the carrier gas of the lunar pyroclastic glass beads. We find that the most water-rich melt inclusion of Hauri et al. (2011) would be in equilibrium with a vapor with pH2O∼3bar and pH2 ∼8bar. We constrain the partial pressures of water and molecular hydrogen in the carrier gas of the lunar pyroclastic glass beads to be 0.0005bar and 0.0011bar respectively. We calculate that batch degassing of lunar magmas containing initial volatile contents of 1200ppm H2O (dissolved primarily as hydroxyl) and 4–64ppm C would produce enough vapor to reach the critical vapor volume fraction thought to be required for magma fragmentation (∼65–75vol.%) at a total pressure of ∼5bar (corresponding to a depth beneath the lunar surface of ∼120m). At a fragmentation pressure of ∼5bar, the calculated vapor composition is dominated by H2, supporting the hypothesis that H2, rather than CO, was the primary propellant of the lunar fire fountain eruptions. The results of our batch degassing model suggest that initial melt compositions with >∼200ppm C would be required for the vapor composition to be dominated by CO rather than H2 at 65–75% vesicularity.