AN EXPERIMENTAL STUDY ON SULFUR PARTITIONING BETWEEN DACITIC MELT AND APATITE AS A FUNCTION OF OXYGEN FUGACITY

Apatite is a minor mineral in magmatic and magmatic-hydrothermal systems and has proven useful in recording magmatic source conditions and evolution. Sulfur (S) partitioning from the silicate melt into apatite (D_S^ap/melt) is strongly controlled by redox conditions, and the S content of apatite has been proposed as a proxy for magmatic S content and oxygen fugacity (fO₂), two variables critical to volcanic eruptions and generating metal sulfide ore deposits. Recent experimental work validated these claims for mafic systems, but no study has assessed S partitioning in intermediate to felsic systems. This is significant as apatite S content has most extensively been utilized in intermediate to felsic magmas, as they are prone to explosive eruptions and host S-rich mineral deposits.

Here we report new experimental data that constrain D_S^ap/melt in a dacitic melt across a range of redox conditions relevant to natural systems. Six apatite crystallization experiments were conducted using a natural dacite at 1000°C, 300 MPa, and log fO₂ varying over four orders of magnitude (ΔFMQ-0.7, ΔFMQ+0, ΔFMQ+0.5, ΔFMQ+1, ΔFMQ+1.75, and ΔFMQ+3.3; where ΔFMQ is log units relative to the fayalite-magnetite-quartz mineral redox buffer). We measured the S contents in the coexisting dacitic glass and apatite to calculate D_S^ap/melt and the in situ oxidation states of S in apatite to quantify the S⁶⁺/ΣS as a function of fO₂. The S content of the dacitic glass and apatite reveals the same trends as those observed in mafic systems; however, we observe a distinct D_S^ap/melt trend in dacitic melts. Whereas D_S^ap/melt increases systematically with fO₂ in mafic systems, D_S^ap/melt is higher overall and more variable in dacitic systems despite a similar systematic increase in S⁶⁺/ΣS in apatite crystallized from both mafic and dacitic melts.

The results indicate different solution and partitioning behavior for S in dacitic melts relative to basaltic melts and demonstrate that D_S^ap/melt values derived from experiments with mafic systems are not directly applicable to more evolved systems. This study sheds light on how the partitioning of S into apatite varies with melt composition and oxidation state and is paramount to the correct utilization and interpretation of using the S content in apatite as a proxy for magmatic conditions.