Sulfur Isotopes in Subduction Systems and the Global Sulfur Cycle

University Of Maine, Orono ME

Investigators

Abstract

Sulfur is one of the most abundant elements in the Earth, is an essential ingredient for life, and plays a pivotal role in geologic and environmental processes. A large part of the global sulfur cycle involves exchange between the surface and interior of the Earth at subduction zones, where one tectonic plate sinks beneath another. As a volatile element, sulfur has the potential to be released into the Earth's mantle during the physical and chemical changes that take place during subduction. Sulfur released during subduction can be re-introduced into the atmosphere from subduction-related volcanic arcs, such as those found along the Pacific rim. While the chemistry of sulfur released at volcanic arcs has been well studied, the mechanism by which sulfur is released during subduction, and the effects of subducted sulfur on the mineralogy of the upper mantle, the volcanic arc, and arc-associated ore deposits (i.e., gold and copper), are poorly constrained. This team's work addresses this knowledge gap in the sulfur cycle by directly examining sulfur-bearing minerals in rocks from ancient subducted plates and deep arc volcanic systems, which have been brought back to the surface of the Earth via tectonic processes. Using a combination of field-based observations and thermodynamic modeling, one of the most important outcomes of this work will be the identification of coupled sulfide-silicate reactions that occur within subducting slabs. The oxidation state of the fluids released during these reactions has important consequences for the redox state of the upper mantle and volcanic arc system. These observations will be coupled with analyses of the sulfur isotopic signature of sulfides from exhumed high-pressure rocks and deep arc crustal cumulates, which will provide a tracer for different sources of sulfur throughout the entire subduction process. A broad range of lithologies, including meta-sediments and meta-igneous rocks, will be analyzed in order to capture the scope of subducted sulfur isotope values. In combining sulfur isotope signatures with the reactions that likely take place in a variety of subducted lithologies, the redox and isotopic composition of sulfur-related fluids that feed arc volcanic systems will be characterized in a way that has never been done before. These linkages will be tested by quantifying the sulfur isotopic signature of sulfides in deep crustal arc cumulates from the Lesser Antilles arc, which provides a novel opportunity to test how differences in subducted sediment across an arc may influence the sulfur isotopic signature in the volcanic arc. This work will provide University of Maine students with access to techniques not currently available in the State of Maine.