Sulfur in Apatite as a Volatile and Redox Tracer in Magmatic and Magmatic-Hydrothermal Systems
Konecke, Brian
2019
Abstract
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 < <1) with respect to apatite that crystallizes from rhyolitic melts under low fO2 conditions (e.g., ≤FMQ), suggesting that the elevated S contents in lunar apatite cannot be explained by fractional crystallization processes alone. Instead, mechanisms involving either several orders of magnitude higher fO2, or metasomatic reactions involving apatite and S and Cl-bearing, F-poor volatile phase(s), is required. In Chapter 4, I 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., DSap/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), relatively reduced ocean island basalts (OIB), and backarc basin basalt (BABB) systems.Subjects
Sulfur Apatite Oxybarometer X-ray absorption near edge structures (XANES) spectroscopy Lunar volatiles Experimental petrology
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