期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：2014
卷号：111
期号：42
页码：15007-15012
DOI：10.1073/pnas.1408118111
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：SignificanceThe hydrogen isotopic ratio of water is commonly used to identify water sources. However, because the Moon is an airless body, the external fluxes of particles and solids that reach its surface should represent an important contribution to its hydrogen budget and thus alter its pristine D/H ratio. To estimate the relative proportions of the solar wind and of the cosmogenic deuterium in the hydrogen budget, the lithium and the hydrogen isotope ratios were measured simultaneously with the Cameca NanoSIMS 50. Analyses demonstrate that all D comes from spallation reactions. On the surface of the soil grains, their D/H ratios indicate that this source of "water" can be ascribed to solar wind implantation and that the chondritic contribution is negligible. Recent data from Apollo samples demonstrate the presence of water in the lunar interior and at the surface, challenging previous assumption that the Moon was free of water. However, the source(s) of this water remains enigmatic. The external flux of particles and solid materials that reach the surface of the airless Moon constitute a hydrogen (H) surface reservoir that can be converted to water (or OH) during proton implantation in rocks or remobilization during magmatic events. Our original goal was thus to quantify the relative contributions to this H surface reservoir. To this end, we report NanoSIMS measurements of D/H and 7Li/6Li ratios on agglutinates, volcanic glasses, and plagioclase grains from the Apollo sample collection. Clear correlations emerge between cosmogenic D and 6Li revealing that almost all D is produced by spallation reactions both on the surface and in the interior of the grains. In grain interiors, no evidence of chondritic water has been found. This observation allows us to constrain the H isotopic ratio of hypothetical juvenile lunar water to {delta}D [≤] -550{per thousand}. On the grain surface, the hydroxyl concentrations are significant and the D/H ratios indicate that they originate from solar wind implantation. The scattering distribution of the data around the theoretical D vs. 6Li spallation correlation is compatible with a chondritic contribution <15%. In conclusion, (i) solar wind implantation is the major mechanism responsible for hydroxyls on the lunar surface, and (ii) the postulated chondritic lunar water is not retained in the regolith.
