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Speciation and Dissolution of Hydrogen in the Proto-Lunar Disk

Despite very high temperatures accompanying lunar origin, indigenous water in the form of OH has been unambiguously observed in Apollo samples in recent years. Such observations have prompted questions about the abundance and distribution of lunar hydrogen. Here, we investigate the related question of the origin of lunar H: is the hydrogen observed a remnant of a much larger initial inventory that was inherited from a wet Earth but partly depleted during the process of origin, or was hydrogen quantitatively lost from the lunar material, with water being delivered to lunar reservoirs via subsequent impacts after the origins sequence? Motivated by recent results pointing to a limited extent of hydrogen escape from the gravity field of the Earth during lunar origin, we apply a newly developed thermodynamic model of liquid-vapor silicates to the proto-lunar disk to interrogate the behavior of H as a trace element in the energetic aftermath of the giant impact. We find that: (1) pre-existing H-bearing molecules are rapidly dissociated at the temperatures considered (3,100-4,200 K) and vaporized hydrogen predominantly exists as OH(v), H(v) and MgOH(v) for nearly the full range of thermal states encountered in the proto-lunar disk, (2) despite such a diversity in the vapor speciation, which reduces the water fugacity and favors hydrogen exsolution from co-existing liquids, the equilibration of the vapor atmosphere with the disk liquid results in significant dissolution of H into proto-lunar magmas, and (3) equilibrium H isotopic fractionation in this setting is limited to < 10 per mil and the terrestrial character of lunar D/H recently inferred should extend to such a precision if liquid-vapor equilibration in the proto-lunar disk is the process that gave rise to lunar hydrogen. Taken together, these results implicate dissolution as the process responsible for establishing lunar H abundances.