Sulfur and the redox evolution of arc magmas

Petrologists have observed for long time that arc magmas are commonly oxidized relative to mid-ocean ridge basalts, but the processes that drive this difference are still debated. The transport of sulfur in and out of arc magmas during subducting slab dehydration, magma degassing, and sulfide mineral precipitation can influence the redox state of magmas. Conversely, the solubility of sulfur in magmas is closely tied to oxidation state, and sulfur can act as a valuable tracer of magma redox evolution. These projects use the geochemistry of primitive melt inclusions to tease apart these processes, yielding valuable insights in magma evolution that have implications for things like volcano gas monitoring, ore deposit formation, and arc magma petrogenesis.

Sulfur in warm subduction zones: a case study in the southern Cascades

This project uses the Lassen volcanic region as a natural labratory to explore how slab-derived sulfur affects the sulfur content, oxidation state, and metal contents of arc magmas. Lassen is located in the southern Cascades, a subduction zone where young, hot oceanic crust is being subducted. We analyze primitive olivine-hosted melt inclusions collected from tephra of Lassen cinder cones for major elments, trace elements (including chalcophile metals such as Cu), volatile elements, S isotopes, and Fe and S speciation. This data set allows us to precisely link singals of slab-derived sulfur to variations in the composition of primitive magmas.

A global perspective on sulfur cycling

At relatively oxidizing conditions, sulfur solubility is several times greater than at conditions similar to mid-ocean ridge settings. This project takes advantage of this relationship by using S contents of primitive, olivine-hosted melt inclusions to place constraints on the oxidation state of primitive arc magmas. Concentrations of sulfur in primitive arc magmas were calculated from a compilation of primitive olivine-hosted melt inclusion compositions compiled in a global literature database. This data set spans 18 subduction zone segments across a range of thermal regimes, and allows us to draw comparisons across arcs globally.

Adventures Outside the Lab



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Mailing Address:
Michelle Muth
Department of Mineral Sciences
National Museum of Natural History
Department of Mineral Sciences
Washington D.C., 20013

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