Sulfur Cycling in the Southern Cascade Arc: Implications for the Sulfur Content, Metal Content, and Oxidation State of Arc Magmas
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Date
2021-11-23
Authors
Muth, Michelle
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Publisher
University of Oregon
Abstract
The behavior of sulfur (S) in volcanic arcs is of fundamental importance to a wide range of geologically and societally relevant processes. Despite this, the influence of subduction on the sulfur content of primitive arc magmas is poorly understood. This is partially because sulfur behavior in arc magmas is challenging to characterize; sulfur is present in two different valence states within silicate melts, and partitions into multiple phases (sulfide, anhydrite, silicate melt, gas). In this work, I address these challenges through the study of high-Mg melt inclusions. The first portion of this dissertation carries out a detailed investigation on the behavior of sulfur within six mafic cinder cones in the southern Cascade Arc. This investigation focuses on the close connections between magma sulfur content, oxidation state, and metal content. Comparisons of δ34S, S and Fe valence state, and melt S content measured within the same suite of inclusions demonstrate that the influence of oxidized slab-derived sulfur in the sub-arc mantle is linked to increases in the oxidation state of arc magmas. In the second portion of this work, applying this framework to metal concentrations in Lassen magmas shows that Pb is dominantly derived from the subducting slab, whereas Mo, Zn, W, and Sn show no clear subduction-related enrichments. Variations in Cu contents between cinder cones are controlled by a combination of ambient mantle heterogeneity and modern subduction influence. In the third portion of this dissertation, I consider arc magma sulfur behavior in a global context through a compilation of published melt inclusion data spanning 115 volcanoes and 29 arc segments. Correlations between S and Cl in these melt inclusions indicate that S is partially derived from the subducting slab. Using models of sulfur content at sulfide saturation, I show that 90% of arc magmas have sulfur contents requiring conditions more oxidizing than the QFM (quartz-fayalite-magnetite) buffer. Collectively, these three chapters demonstrate that subduction strongly influences the sulfur content of arc magmas, and that the transfer of sulfur from the subducting slab into the mantle source of arc magmas can be an important control on both the oxidation state and metal content of primitive arc magmas. This dissertation includes previously published and unpublished co-authored material.
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Keywords
Oxygen Fugacity, Subduction Zones, Sulfur