Collaborative Research: Tracking Novel Metal Isotope Signatures during Subduction Metamorphism

University Of Massachusetts Lowell, Lowell MA

Investigators

Abstract

Subduction is a key geologic process governing the Earth’s chemical evolution. Through this process, sediments bearing the chemical signatures of low-temperature interaction with the hydrosphere and atmosphere are delivered, along with ocean crust altered by interaction with seawater, to the Earth’s deep interior. With increasing depth and pressure, metamorphic changes to the subducting slab’s mineralogy lead to the release of fluids and, in some cases, melts, enriched in some elements over others. These materials may return to the Earth’s surface via the route of arc volcanism, leaving behind a chemically altered slab that continues to sink deeper into the mantle. The research broadly aims to better understand the chemical exchange that occurs between the slab and overlying mantle during subduction metamorphism using the molybdenum (Mo) and thallium (Tl) isotope tracer systems. Isotopes of Mo and Tl experience strong fractionation in the oceans, leading to distinct isotope signatures in sediments and altered ocean crust that are ultimately transferred to subduction zones. While recent research on the Mo and Tl isotope systems in volcanic arc rocks indicate that Tl isotopes match subduction zone inputs well, there is a large mismatch between the Mo isotope composition of most arcs and subduction zone inputs. The work will focus on the origins of the divergent behaviors of the Mo and Tl isotope systems and provide new understanding of the deep Earth cycling of these elements. The work uses two well-characterized suites of high pressure-low temperature (HPLT) metamorphic rocks as analogs for the chemical processes and mineralogical changes that occur in sedimentary rocks during subduction. The Schistes Lustres and related rocks from the western Alps exhibit a metamorphic gradient that ultimately reaches pressures approaching 3 GPa, but prior work has shown only limited mobile-element loss. The Catalina Schist from California represents warmer subduction metamorphic conditions and shows considerable mobile element loss. The bulk-rock Mo and Tl isotope compositions of the two sample suites will be determined in order to ascertain whether these isotope systems experience fractionation during HPLT metamorphism. In situ analysis via LA-ICP-MS will determine which minerals host these elements and how they are redistributed during progressive metamorphism. Isotope analysis of purified mineral splits will determine whether isotopic fractionation occurs between mineral phases. 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.