SULFIDE, SULFITE AND SULFATE IN APATITE: A NEW OXYBAROMETER

Apatite is a ubiquitous phase in terrestrial and extraterrestrial magmatic and magmatic-hydrothermal systems and incorporates redox sensitive elements such as Fe, Mn, and S. The observation that apatite in oxidized terrestrial systems is commonly enriched in S suggests that sulfate, S⁶⁺, replaces cations such as P and Ca in the apatite structure via several independent substitutions such as P⁵⁺ + Ca²⁺ = S⁶⁺ + Na⁺ and 2P⁵⁺ = S⁶⁺ + Si⁴⁺. Apatite in reduced terrestrial systems commonly contains low concentrations of S, which suggests that apatite does not incorporate sulfide, S^2-. However, some apatite grains from lunar systems, where oxygen fugacity is extremely low relative to terrestrial systems, have been reported to contain elevated concentrations of S, suggesting that apatite may incorporate S^2-. Here, we experimentally assessed the ability for apatite in equilibrium with a mafic silicate melt to incorporate different oxidation states of S at 1000 °C and 300 MPa as a function of oxygen fugacity from log(fO2) = FMQ-1, FMQ, FMQ+0.25, FMQ+0.8, FMQ+1.2 and FMQ+3, where FMQ = fayalite-magnetite-quartz solid buffer. We used electron probe microanalyses (EPMA) to measure S concentrations in apatite, and micro-X‑ray absorption near-edge structures (μ-XANES) spectroscopy at the S K-edge to measure in situ the oxidation state of S in apatite. The results reveal that apatite/melt partition coefficients for S increase systematically as oxidation state increases from FMQ-1 to FMQ+3, and that apatite incorporates S^2-, S⁴⁺, and S⁶⁺, with concentrations of S⁶⁺ >> S⁴⁺> S^2-.The integrated S⁶⁺/ΣS peak area ratios vs. ΔFMQ results exhibit a sharp transition from low S⁶⁺/ΣS to high S⁶⁺/ΣS indicating that the oxidation state of S in apatite can be used as an oxybarometer. This redox range is particularly relevant for MORB systems, and arc related magmas, which can oxidize upon degassing of a S-bearing fluid. Considering that apatite is commonly a near-liquidus phase in arc magmas, this new oxybarometer allows users to determine the pre-degassed oxygen fugacity of magmatic systems. New μ-XANES data will be presented for S in lunar apatite where S^2-, S⁴⁺, and S⁶⁺are present, revealing that late-stage redox processes affected lunar magmas.