The Isotopic Composition of Molecular Water Formed by Hydrogen Reduction of Iron Oxides: A Proxy for Endogenous Water Formed From Solar Wind Implantation

Many workers have successfully utilized a variety of lunar materials to infer the sources and processing history of volatiles on the Moon. In situ techniques via NanoSIMS have enabled a plethora of hydrogen isotope measurements of isolated grains in regolith and apatite within igneous rocks, revealing an extraordinarily large range of δD values from ~ -900 to +1000‰. The lowest of these δD values are related to the incorporation of solar wind hydrogen, which is pure ¹H. Exposure to solar wind results in structural damage to crystalline phases, forming ubiquitous tens-to-hundreds of nanometer thick amorphous rims on mature regolith grains. Solar wind implantation of H⁺ is also is also thought to result in the formation of nanophase Fe⁰ (npFe), a quantitative metric of surface exposure time (e.g., Is/FeO). The formation of npFe can be generally described via the reaction: Fe²⁺O + H₂ = Fe⁰ + H₂O, resulting in the formation of endogenous molecular water reservoirs.