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The volatile contents and D/H ratios of the Apollo 15 lunar volcanic glasses


IPGP - Campus Jussieu


Séminaires généraux de l’IPGP

Salle Bleue

Alberto Saal

Brown University, USA

The general consensus is that the Moon formed and evolved through a single or series of catastrophic heating events in which most of the highly volatile elements, especially hydrogen, were evaporated away. That notion has changed with the new report showing evidences of indigenous water in lunar volcanic glasses. Because these glasses are the most primitive melts erupted on the surface of the satellite, this result represents the best evidence for the presence of a deep source within the Moon relatively rich in volatile. Here we report new volatile data (C, H2O, F, S, Cl) for over 200 individual Apollo 15 lunar glasses with composition ranging from very-low to high Ti contents (sample 15427,41; 15426,138; 15426,32). Our new SIMS detection limits (~0.15 ppm C; ~0.4 ppm H2O, ~0.05 ppm F, ~0.21 ppm S, ~ 0.04 ppm Cl by weight determined by the repeated analysis of synthetic forsterite located on each sample mount), represent at least 2 orders of magnitude improvement over previous analytical techniques. After background correction the volatile contents have the following ranges: C 0-0.28± 0.11 ppm is within background; 0-70 ± 0.4 ppm for H2O; 1.6-60 ± 0.1 ppm for F; 58-885 ± 1.3 ppm for S; and 0-3 ± 0.02 ppm for Cl. Our new values represent an increase in the volatile concentrations by a factor of 2 from previously reported data. Two outstanding features of the data are the significant correlation among H2O, Cl, F and S contents, and the clear relationship between the volatile and the major element contents of the glasses. The data support the hypothesis that there were significant differences in the initial volatile content, and/or the mechanism of degassing and eruption among these glasses was different. Most importantly, the data suggest that the measured H2O is indigenous to the Moon. Our results suggest that, contrary to the prevailing ideas, the bulk Moon is not uniformly depleted in highly volatile elements, and the presence of water, in particular, must be included to constrain models for the thermal and chemical evolution of the Moon.s interior.