THE OXIDATION STATE OF SULFUR IN APATITE AS A FUNCTION OF THE REDOX CONDITIONS

The oxygen fugacity (ƒO₂) of magmatic systems is a fundamental variable that influences crystallization and degassing processes, as well as ore metal ratios in porphyry ore deposits [1]. Apatite—commonly Ca₅(PO₄)₃(F, Cl, OH)—is a resistant, ubiquitous mineral in magmatic systems and can contain up to several thousand μg/g of S [2]. In this study, the oxidation state of S in apatite as a function of ƒO₂ is investigated using X-ray absorption near-edge structures (XANES) spectroscopy at the S K-edge. Experimental apatites crystallized from lamproitic melts over a broad range of ƒO₂ and sulfur fugacities (fS₂) were measured via S XANES. Peaks energies corresponding to sulfate S⁶⁺ (~2482 eV), sulfite S⁴⁺ (~2478 eV) and sulfide S^2- (~2470 eV) were observed in apatite. The integrated S⁶⁺/S_Total (i.e., S_Total = S⁶⁺ + S⁴⁺ + S^2-) peak area ratios provide information about the proportions of S oxidation states in the apatite grains. Apatite crystallized at intermediate (FMQ+1.5) and oxidizing conditions (FMQ+3) are dominated by S⁶⁺ with a minor contribution of S⁴⁺, where the integrated S⁶⁺/S_Total peak area ratios = 0.958 and 0.963, respectively. Apatites crystallizing at reduced conditions (FMQ+0) contain predominantly S^2- and lesser amounts of S⁶⁺, where the integrated S⁶⁺/S_Total peak area ratios = 0.168. To our knowledge, this observation makes apatite the first mineral to incorporate reduced (S^2-), intermediate (S⁴⁺), and oxidized (S⁶⁺) S in variable proportions as a function of the fO₂ of the system. We emphasize that the strong dependence of the S oxidation state in apatite as a function of fO₂ is also coupled with changing S content of apatite and co-existing melt (i.e., with changing ƒS₂), resulting in a complex correlation between [a] apatite-melt (or fluid) partitioning, [b] redox conditions and [c] the melt and/or fluid composition, making the application of previously determined partitioning coefficient debatable. Upon calibration over a range of geologically relevant T-P-X- ƒO₂- ƒS₂, S-in-apatite can serve as a powerful oxybarometer to quantify fO₂. [1] Candela & Piccoli, 2005, Economic Geology. [2] Parat et al., 2011, Rev. Mineral. Geochemistry.