A GIANT IMPACT ORIGIN FOR ELEVATED D/H OF THE LUNAR MANTLE

Recent studies have suggested an origin of lunar water from carbonaceous chondrites rather than comets. Greenwood et al. (2011) suggested at least two sources of lunar water based on their study of apatite in Apollo samples: a component with a low D/H and another component with a high D/H. They suggested cometary input to the Moon as a possible source of the high D/H waters. They also suggested three possible origins for the source of low D/H water: 1) a component similar to the terrestrial mantle (similar to chondrite water) that became elevated by the admixture of cometary water or meteoritic/interstellar organic compounds, 2) or a component with dD ~ -200‰, similar to putative impact-derived apatite in basalt 14053, or 3) a component with dD ~ +300‰, similar to apatite in a plutonic highlands alkali anorthosite. Greenwood et al. (2011) postulated that the elevated D/H of the lunar highlands sample could have originally been terrestrial-like, but became elevated during hydrodynamic escape of hydrogen during the giant impact event. Here we provide evidence from apatite grains and olivine-hosted melt inclusions in Apollo samples for an elevated D/H of the lunar mantle, and propose that the elevated D/H of the lunar mantle results from hydrodynamic escape of hydrogen in the aftermath of the giant impact event. Apatite from highlands samples and mare basalts consistently have elevated D/H with minimum dD of ~+200-+300‰. The D/H of the Moon before the giant impact may have been similar to that of the current Earth’s mantle. Values of dD for lunar samples lower than ~+200-+300‰ are likely affected by addition of hydrogen from the hydrogen-rich lunar regolith during impact, as suggested for basalt 14053. Highly elevated D/H of lunar materials could be related to assimilation of D-rich materials in the lunar regolith, such as cometary/meteoritic/interstellar organics and cometary ice, or result from degassing of H₂, during lunar volcanism. The magnitude of the H isotope anomaly correlates with those of Cl and Zn for lunar materials, consistent with hydrodynamic escape of gaseous species to explain an elevated D/H of the lunar mantle.