Where Did the Water Go? Modeling the Diffusion of Hydrogen in Lunar Apatite

The Moon was considered to be dry in comparison to the Earth, until reports on the volatile nature of lunar rocks have provided opposing evidence. Recent studies have found lunar water existing in hydrous mineral apatite, Ca5(PO4)3(OH,F,Cl). One of the first, showed quantitative ion microprobe measurements of Lunar Basalt 14053, a late-stage apatite, providing concentrations of hydrogen, chlorine and sulfur. The values were comparable to terrestrial values, suggesting that the Moon could be less dry than prior beliefs and thus more like Earth. The widespread occurrence of apatite in a variety of lunar rocks has the potential to reveal the volatile inventory of the moon. This new finding brings into question how water moves through the mineral during cooling of lunar magmas. Apatite is known to equilibrate with its surroundings in high-temperature and high-pressure environments. Therefore the diffusion properties of apatite would be a valuable asset. By dehydroxylating single crystal and powder apatite at high temperatures (800C-1200C) and measuring H and D/H with a mass spectrometer and an ion microprobe, we can obtain a diffusion coefficient for hydrogen and deuterium in apatite. This will allow us to determine constraints on the cooling rates of lunar magmas. The diffusivity of hydrogen in apatite will be key in understanding the history and supply of lunar water, which plays an important role in the evolution of planetary bodies.