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The Lunar Apatite Paradox

Science  25 Apr 2014:
Vol. 344, Issue 6182, pp. 400-402
DOI: 10.1126/science.1250398

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Wetted Apatite

The long-running story of the dry Moon was rewritten a few years ago when hydrogen-bearing glass spherules were discovered. The highest water contents are found in lunar apatite, at levels suspiciously comparable to the water content of Earth apatites. Boyce et al. (p. 400, published online 20 March; see the Perspective by Anand) now show that the water content of lunar apatite is not a reliable indicator of the abundance of water in mare basalts. The existence of apatite with high water content is an almost inevitable consequence of the loss of tiny amounts of fluorine-rich apatite from a melt and replacement by hydrogen and is thus no indication of a “wet” Moon.

Abstract

Recent discoveries of water-rich lunar apatite are more consistent with the hydrous magmas of Earth than the otherwise volatile-depleted rocks of the Moon. Paradoxically, this requires H-rich minerals to form in rocks that are otherwise nearly anhydrous. We modeled existing data from the literature, finding that nominally anhydrous minerals do not sufficiently fractionate H from F and Cl to generate H-rich apatite. Hydrous apatites are explained as the products of apatite-induced low magmatic fluorine, which increases the H/F ratio in melt and apatite. Mare basalts may contain hydrogen-rich apatite, but lunar magmas were most likely poor in hydrogen, in agreement with the volatile depletion that is both observed in lunar rocks and required for canonical giant-impact models of the formation of the Moon.

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