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

Sulfur;Apatite;Oxybarometer;X-ray absorption near edge structures (XANES) spectroscopy;Lunar volatiles;Experimental petrology;Geology and Earth Sciences;Science;Earth and Environmental Sciences

Konecke, BrianMoore, Gordon ;

University of Michigan

关键词: Sulfur;    Apatite;    Oxybarometer;    X-ray absorption near edge structures (XANES) spectroscopy;    Lunar volatiles;    Experimental petrology;    Geology and Earth Sciences;    Science;    Earth and Environmental Sciences;   

Others  :  https://deepblue.lib.umich.edu/bitstream/handle/2027.42/149929/bkonecke_1.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

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.