Effects of pH2O, pH2 and fO2 on the diffusion of H-bearing species in lunar basaltic liquid and an iron-free basaltic analog at 1 atm.

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]