Congratulations to Brian Konecke who defended his dissertation on December 14, 2018
Advisor: Adam Simon
Sulfur is the third most abundant volatile in terrestrial magmatic systems, and the oxidation state(s) and behavior of S are intrinsically linked to oxygen fugacity (fO2). However, the quantification of the redox conditions and distribution (i.e., transport and storage) of S during the evolution of magmatic systems remains an enigma. Considering that apatite can be a near liquidus phase in silicate melts, the intracrystalline 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.
Chapter II uses micro X-ray absorption near edge structure spectroscopy (μ-XANES) at the S K-edge to measure the oxidation state(s) of S in experimentally grown apatite and co-existing melt. We demonstrate 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 a 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.
Chapter III attempts to reconcile the observation that apatite crystallizing from late-stage lunar felsic (rhyolitic) melts contain relatively elevated concentrations of S (up to ~430 µg/g S), despite crystallizing from a reduced, anhydrous, and evolved melt containing <100 µg/g S. Apatite crystallization experiments equilibrated at conditions relevant to late-stage lunar magmatism indicate that S behaves incompatibly (e.g., DSap/m <<1) with respect to apatite that crystallizes in low fO2 conditions (e.g., ≤FMQ), suggesting that the elevated S contents in lunar apatite cannot be explained via fractional crystallization processes alone.
Chapter IV performed apatite crystallization experiments to constrain the influence of fO2 and bulk S contents on the oxidation states of S in apatite, and the distribution of S between apatite and melt (i.e., DS,ap/m). The experimental results indicate that the integrated S6+/ΣS peak area ratios, centroid energies (eV), and DSap/m increase systematically with increasing fO2. From this dataset, an empirical S-in-apatite oxybarometer was developed and is applicable to mafic systems such as mid ocean ridge basalt (MORB) and relatively reduced ocean island basalts (OIB) and backarc basin basalt (BABB) systems.