Behaviour of trace metals in the solid earth and atmosphere during subduction and volcanism

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The upper surface of the Earth consists of tectonic plates. Where they collide, one plate is consumed, creating a flux of material to the deeper earth, as well as the addition of new magma to the surface of the other plate, expressed as a chain of volcanoes (arcs). The flux of elements into the deeper earth during subduction, and their return to the surface by volcanism, makes an important geochemical cycle critical to the evolution of the earth. Sulfur (S) and other elements that pair with it (chalcophile metals) are key components in this cycle that affect the composition and evolution of the atmosphere, and the endowment of the crust in metals required as nutrients by living organisms, or exploited by humans for technological use. This research program investigates the sequestration of S and allied trace metals in subducted rocks, and their release to overlying mantle and crust by fluids, melts and gases during plate convergence. The identity and stability of S-bearing minerals (sulfides) in the presence of fluids is investigated by laboratory experiments over the pressure-temperature regime experienced by subducted Ca-, Al- and Fe-rich rock compositions in the earth. The partitioning of S and chalcophile metals (As, Zn, Pb, Cu, Cd, Mo) into fluids measured during the experiments can be applied to natural rocks to understand how mass transfer deep beneath arcs can either sequester elements into the deeper earth, enrich the crust with ore metals, or control the composition of volcanic gases emitted to the atmosphere. The results of such experiments elucidate the stability and host for S and chalcophile element transfer expected from the subducted crust into the deep mantle as functions of many variables. The detailed examination of behaviour S and allied chalcophiles in fluids and solids will help answer a long standing question as to how the mantle beneath subduction zones can oxidize, or produce magmas with oxidized gases, controlling the evolution of oxygen in the atmosphere over time. The return flux of magma by subduction to the upper plate also emits gases. The output of toxic trace metals in these gases emitted to the atmosphere by volcanism will be investigated in the laboratory with a new apparatus that simulates natural volcanic vents. The apparatus will be used to measure the precipitation of trace metals from cooling volcanic gases into solid forms in vents and on volcanic ash particles. The overarching goal will be to isolate the variables in how metals are transported in a gas phase, in what species and what possible fractionations ensue during low pressure metal transport. The impact of these measurements will inform a better understanding of volcanic degassing processes and the formation of ore deposits and help quantify the toxicity of modern eruptions. Ultimately we can test if volcanic eruptions in the past loaded the surface environment with enough trace metals to influence living organisms or cause mass extinctions.