Sulfur in Apatite as a Volatile and Redox Tracer in Magmatic and Magmatic-Hydrothermal Systems

Sulfur (S) is the third most abundant volatile element in terrestrial magmatic systems, where the oxidation state(s) and behavior of S are intrinsically linked to oxygen fugacity (fO2). However, the quantification of the oxidation states and distribution (i.e., transport and storage) of S during the evolution of magmatic systems remains poorly understood. Considering the ability of the mineral apatite to crystallize from silicate melts, the intra-and-inter-crystalline zonation with respect to S content and oxidation state(s) of S-in-apatite may serve as a proxy to reconstruct redox and degassing processes in magmatic environments. In Chapter 2, I used micro X-ray absorption near edge structure spectroscopy (μ-XANES) at the S K-edge to measure the formal oxidation state(s) in experimentally grown apatite and co-existing lamproitic melt. The study demonstrates-for the first time-that apatite incorporates three oxidation states of S (S2-, S4+, and S6+) in variable proportions, as a function of the prevailing fO2 of the system that spans the complete transition of sulfide (S2-) to sulfate (S6+) in the silicate melt. A new technique involving the integrated peak area ratios of S2-, S4+ and S6+ (e.g., S6+/ΣS) in apatite was developed to empirically correlate the proportions of sulfur oxidation states in apatite to the redox conditions of the system, thus serving as the foundation for an empirical oxy-sulfo-barometer. In Chapter 3, I attempt to reconcile the observation that apatite crystallizing from late-stage lunar felsic (rhyolitic) melts contains relatively elevated concentrations of S (up to ~430 µg/g S), despite crystallizing from a reduced and anhydrous melt containing < 100 µg/g S. Apatite crystallization experiments preformed at conditions relevant to late-stage lunar magmatism indicate that S behaves incompatibly (e.g., DSap/m <