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We have experimentally determined the diffusivity of water in a representative lunar basaltic liquid composition (LG) and in an iron-free analog of a basaltic liquid (AD) at the low water concentrations and low oxygen fugacities (f O 2) relevant to the eruption of lunar basalts. Experiments were conducted at 1 atm and 1350 °C over a range of p H 2 / p H 2 O from near zero to ∼10 and a range in f O 2 spanning ∼9 orders of magnitude (from 2.2 log units below the iron-wüstite buffer, IW−2.2, to IW+6.7). The water concentrations measured in our quenched experimental glasses by secondary ion mass spectrometry (SIMS) and Fourier transform infrared spectroscopy (FTIR) vary from a few ppm to ∼430 ppm. Water concentration gradients in the majority of our AD experiments are well described by models in which the diffusivity of water (D w a t e r ∗) has a constant value of ∼2 × 10−10 m2/s, while our LG results indicate that D w a t e r ∗ in LG melt has a constant value of ∼6 × 10−10 m2/s under the conditions of our experiments. Water concentration gradients in hydration and dehydration experiments that were run simultaneously in H 2 /CO 2 gas mixtures are well described by the same D w a t e r ∗ , and water concentrations measured near the melt-vapor interfaces of these experiment pairs are approximately the same. These observations strongly support an equilibrium boundary condition for our experiments containing >70 ppm H 2 O. However, dehydration experiments into nominally anhydrous CO 2 , N 2 , and CO/CO 2 gas mixtures leave some scope for the importance of kinetics during dehydration of melts containing less than a few 10′s of ppm H 2 O. Comparison of our results with the modified speciation model (Ni et al., 2013) in which both molecular water and hydroxyl are allowed to diffuse suggests that we have resolved the diffusivity of hydroxyl (D O H) in AD and LG melts. Our results support a positive correlation between D O H and melt depolymerization. Best-fit values of D w a t e r ∗ for our LG experiments vary within a factor of ∼2 over a range of p H 2 / p H 2 O from 0.007 to 9.7 and a range of log f O 2 from IW−2.2 to IW+4.9. The relative insensitivity of our best-fit values of D w a t e r ∗ to variations in p H 2 suggests that H 2 diffusion did not control the rate of degassing of H-bearing species from the lunar glasses of Saal et al. (2008); however, we cannot rule out a role for molecular H 2 diffusion under lower-temperature and/or higher-pressure conditions than explored in our experiments. The value of D w a t e r ∗ chosen by Saal et al. (2008) for modeling the diffusive degassing of the lunar volcanic glasses is within a factor of ∼2 of our measured value in LG melt at 1350 °C. By coupling our LG results at 1350 °C with an activation energy of 220 kJ/mol (Zhang et al., 2017), we obtain the following Arrhenius relationship, which can be used to model syneruptive diffusive water loss from lunar melt beads: D w a t e r ∗ (m 2 / s) = 7.2 × 1 0 - 3 e x p - 2.6 × 1 0 4 T (K) . [ABSTRACT FROM AUTHOR]

We report the solubility of water in Apollo 15 basaltic “Yellow Glass” and an iron-free basaltic analog composition at 1 atm and 1350 °C. We equilibrated melts in a 1-atm furnace with flowing H 2 /CO 2 gas mixtures that spanned ∼8 orders of magnitude in f O 2 (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 p H 2 / p H 2 O (from 0.003 to 24). Based on Fourier transform infrared spectroscopy (FTIR), our quenched experimental glasses contain 69–425 ppm 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 p H 2 O in the furnace atmosphere and is independent of f O 2 and p H 2 / p H 2 O; (3) the solubility of water is very similar in both melt compositions; (4) the concentration of H 2 in our iron-free experiments is <∼4 ppm, even at oxygen fugacities as low as IW−2.3 and p H 2 / p H 2 O as high as 11; (5) Secondary ion mass spectrometry (SIMS) analyses of water in iron-rich glasses equilibrated under variable f O 2 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 p H 2 O ∼ 3 bar and p H 2 ∼ 8 bar. We constrain the partial pressures of water and molecular hydrogen in the carrier gas of the lunar pyroclastic glass beads to be 0.0005 bar and 0.0011 bar respectively. We calculate that batch degassing of lunar magmas containing initial volatile contents of 1200 ppm H 2 O (dissolved primarily as hydroxyl) and 4–64 ppm C would produce enough vapor to reach the critical vapor volume fraction thought to be required for magma fragmentation (∼65–75 vol.%) at a total pressure of ∼5 bar (corresponding to a depth beneath the lunar surface of ∼120 m). At a fragmentation pressure of ∼5 bar, the calculated vapor composition is dominated by H 2 , supporting the hypothesis that H 2 , 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 >∼200 ppm C would be required for the vapor composition to be dominated by CO rather than H 2 at 65–75% vesicularity. [ABSTRACT FROM AUTHOR]