T241 Experimental and Petrologic Investigation of Halogens, Sulfur, and Other Volatile Species in Igneous Systems: In Honor of Jim Webster

Friday, 30 October 2020
10:00 AM - 2:00 PM
Meeting Rooms

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

Paper number: T241-9
Program: Technical Sessions
Time: 11:50 AM – 12:05 PM

Authors

Abstract

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 (DSap/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 DSap/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 DSap/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 DSap/melt trend in dacitic melts. Whereas DSap/melt increases systematically with fO2 in mafic systems, DSap/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 DSap/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.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-350750

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