Résumé
We consider high-S apatites to be apatites with > or =0.7 wt.% SO (sub 3) . High-S apatites of magmas with rhyolitic melt are of particular interest because experimental data suggest that such apatites are 'over enriched' in S relative to equilibrium crystal-melt partitioning processes. In Pinatubo dacite, 29% of all investigated apatites (N=69) include high-S concentrations. Conversely, the dominant apatite populations of silicic oxidized magmas are medium- (0.3-0.6 wt.% SO (sub 3) ) and low-S (<0.3 wt.% SO (sub 3) ) apatites (also at Pinatubo), which are consistent with crystallization from melt at or below S saturation at various temperatures. Explanations for high-S apatites range from inheritance (from mafic magmas or country rock) to S exchange between apatite and anhydrite as a consequence of close petrographic association of both phases in systems containing anhydrite (e.g. at Pinatubo). The compositional fingerprint (e.g. REE, Sr) ties high-S apatites to the same melt environment as the dominant populations, and thus makes inheritance unlikely. Finding low-S apatite inclusions in anhydrite in conjunction with the lack of consistent rimward enrichment of S of anhydrite-hosted apatites, argues against S exchange between these two phases. To generate high-S apatite, we envision a process that we call 'fluid-enhanced crystallization,' whereby the high-S signal is controlled by the presence of a S-rich fluid adjacent to a growing apatite while other compositional characteristics are mostly controlled by the silicate melt. This process would be consistent with observed reverse S zonation towards the rim, small scale and strong changes in S content within single grains, occurrence of low- to high-S apatites as inclusions together in single host minerals, and compositional similarity among low- to high-S apatites. At Pinatubo the existence of such S-rich fluid is corroborated by S isotopes of anhydrite as well as growth features of anhydrite. A suitable source for S-rich fluids would be fluids derived from degassing underplated mafic magmas.