Sulfur speciation in lunar apatite

Apatite incorporates several volatile elements (including S, as SO_4 ^(2-)) and can provide a record of magmatic volatile evolution. Recent measurements of volatiles in apatite from Apollo sample 14053.241 revealed 300-450 ppm S. Although many lunar melts have sufficient S for sulfide saturation, the observed S content of lunar apatite is surprising because lunar samples (especially 14053) are highly reduced (≤ IW) and are thus expected to contain little SO_4 ^(2-). One possibility is that there are micro-environments in late-stage lunar melts that are more oxidized than one would infer from conditions recorded by other components of these rocks. Alternatively, it may be that S^(2-) substitutes for F+Cl+OH in lunar apatite: S^(2—)bearing apatite has been synthesized, but to our knowledge has not been observed in nature.