The use of volatile-bearing (H2O-Cl-F-C-S) minerals as proxies for constraining the volatile content of lunar magmas remains controversial. In this study, we apply published F-Cl-S signature trends in apatite from lunar mare basalts, new experimentally determined apatite-melt partition coefficients for S, and thermodynamic modeling to demonstrate unequivocally that fractional crystallization of lunar silicate melt could not have produced the observed S signatures of lunar apatite. Instead, metasomatic (cryptic) alteration by a S-Cl–bearing, F-poor volatile phase plausibly explains the volatile element zoning in lunar apatite, consistent with some lunar magmas being more volatile rich than previously suggested.

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