J. W. Boyce¹, S. M. Tomlinson¹, F. M. McCubbin², J. P. Greenwood³ and A. H. Treiman⁴

¹Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, Los Angeles, CA 90095, USA.
²Institute for Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA.
³Department of Earth and Environmental Sciences, Wesleyan University, 265 Church Street, Middletown, CT 06459, USA.
⁴Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, TX 77058–1113, USA.

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.

Reference
Boyce JW, Tomlinson SM, McCubbin FM, Greenwood JP and Treiman AH (2014) The Lunar Apatite Paradox. Science 344:400.
[doi:10.1126/science.1250398]
Reprinted with permission from AAAS

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