CAREER: Tracing sulfur in subducting slabs with apatite oxybarometry

Tectonic plate boundaries are important loci of mass transfer in the Earth system. The transfer of volatile elements from Earth’s deep interior to its surface and atmosphere at subduction zones enables a habitable planet. Magmas erupted at arc volcanoes, located at subduction zones, are more oxidized than magmas erupted at mid-ocean ridges. Arc volcanoes are the basis of the continental crust and host economic ore deposits. Quantifying the processes that lead to the oxidation of magmas in subduction zones is then critical for understanding the formation of continents and ores. This project will investigate the hypothesis that the transfer of sulfur throughout the subduction system is responsible for the elevated oxidation state of arc magmas. High-pressure experiments will be executed to measure the behavior of sulfur in rocks at subduction zone conditions. Experimental results will be used to inform observations of natural subduction-related rocks. This project includes research and mentoring experiences for a postdoctoral associate, a PhD student and two undergraduate students. Project results will be assimilated into new open access Earth science educational content. A hybrid capstone outreach event will be held to disseminate research outcomes to the public. This study will examine the hypothesis that the oxidized signature of arc magmas is due to the transfer of sulfate from metamorphosed oceanic crust on the down-going slab to the overlying arc mantle. High-pressure experiments will be executed to calibrate a new proxy for the oxidation state of subduction-related rocks based on the incorporation and valence state of sulfur in the mineral apatite. Experimental apatite will be co-crystallized with sulfide or sulfate phases such that the behavior of sulfur in apatite can be used to provide indirect information on the relative stabilities of sulfide vs. sulfates in subducted oceanic crust. Results from the experiments will be applied to determine the oxidation states of suites of apatite-bearing metabasites to determine if subducting slabs record an increase in oxidation state concurrent with the increase in dissolved oxygen in the deep oceans and rise in seawater sulfate in the Early Phanerozoic. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.