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

1,2,3M.E.Newcombe, 1J.R.Beckett,1M.B.Baker, 4S.Newman,1Y.Guan,1J.M.Eiler1E.M.Stolper
Geochmica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.05.033]
1Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
2Lamont Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964, USA
3Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington, DC 20015
4Bay Area Air Quality District, 375 Beale St., Suite 600, San Francisco, CA 94105
Copyright Elsevier

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 (fO2) relevant to the eruption of lunar basalts. Experiments were conducted at 1 atm and 1350 °C over a range of pH2/pH2O from near zero to ∼10 and a range in fO2 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 (Dwater∗ ) has a constant value of ∼2×10–10 m2/s, while our LG results indicate that Dwater∗ 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 H2/CO2 gas mixtures are well described by the same Dwater∗ , 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 H2O. However, dehydration experiments into nominally anhydrous CO2, N2, and CO/CO2 gas mixtures leave some scope for the importance of kinetics during dehydration of melts containing less than a few 10’s of ppm H2O. 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 (DOH ) in AD and LG melts. Our results support a positive correlation between DOH and melt depolymerization. Best-fit values of Dwater∗ for our LG experiments vary within a factor of ∼2 over a range of pH2/pH2O from 0.007 to 9.7 and a range of logfO2 from IW–2.2 to IW+4.9. The relative insensitivity of our best-fit values of Dwater∗ to variations in pH2 suggests that H2 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 H2 diffusion under lower-temperature and/or higher-pressure conditions than explored in our experiments. The value of Dwater∗ 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:

Dwater∗(m2/s)=7.2×10-3exp-2.6×104T(K) .

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