APATITE AS A RECORDER OF CRYPTIC VOLATILE PROCESSES IN MAGMAS

Apatite is a ubiquitous igneous accessory mineral found in environments as varied as rhyolitic super-eruptions and lunar basalts. Apatite is a mineralogical repository for all of the major volatile phases, OH, F, Cl, S and CO₂ and has thus played an important role in our understanding of the volatile evolution of magmas. Apatite is an important compliment to records in melt and fluid inclusions, particularly as apatite is relatively early in the granite crystallization sequence, refractory, resistant to post-crystallization alteration, and can be found as both an igneous and detrital sedimentary mineral. Concentrations of F, Cl and OH occupying the hydroxyl group site have long been used to interrogate volatile behaviors in igneous systems. To a lesser extent S, as a coupled substitution for P, has been posited to record some information about magma redox behaviors. Recent work has shown that apatite grains are a faithful recorder of the S isotopic composition of dissolved sulfate in magmas from which they crystallize and contain unique information about the redox evolution of magmas during crystallization and degassing. This presentation will summarize progress on interpreting coupled apatite S isotopic ratios collected via Secondary Ionization Mass Spectrometry and S concentration data collected via Electron Microprobe from plutons, volcanic units and detrital records, which demonstrate variations of several permil within single apatite crystals and between crystals within single granite hand samples.