Southernmost Patagonia: The Connection Between Magmatism, Subduction Zone, Slab Window and Sub-continental Mantle Lithosphere

Brown University, Providence RI

Investigators

Abstract

The Earth's mantle represents approximately 82% by volume and 68% by mass of the Earth, and therefore, it influences the movement of tectonic plates, earthquakes and volcanism on the surface. A fundamental step to understand the composition and nature of the Earth's mantle from the study of basaltic lavas (partial melts of the mantle) is to establish the budget and distribution of volatiles (hydrogen, carbon, fluorine, chlorine, sulfur) in the Earth's interior. Volatiles influence mantle melting, magma crystallization, magma migration and volcanic eruption, and their abundances and spatial distribution provide important constraints on models of mantle flow and temperature. Moreover, volatiles are key constituents of the Earth's atmosphere and oceans and, thus, constraining the cycling of volatiles between the Earth's interior and surface is of fundamental importance to understand the long-term evolution of our planet, as they fundamentally affect plate tectonics, climate, and habitability. The project will support an African-American female graduate student, an Indian research scientist and a Hispanic Professor at Brown University. It promotes the collaboration with geochemists from nine institutions from three different countries: USA, Argentina and Chile, and will enhance the infrastructure for research and education at Brown University, Oregon State University and Woods Hole Oceanographic Institution. Communication of results will occur through: 1) peer-reviewed journals, presentations at conferences and invited university lectures, 2) hands-on science learning activities for local elementary and high school classes, and 3) outreach to the general audience through public lectures. Diverse hypotheses have been proposed for the origin of the magmatism in southern Patagonia, from flux melting of the mantle wedge during devolatilization of the subducted Nazca and Antarctic plate, to adiabatic decompression melting of a carbonated and hydrous asthenosphere upwelling through the slab window, to melting of a volatile-rich metasomatized subcontinental lithospheric mantle. All proposed hypotheses invoke the role of volatiles. Surprisingly, data on the volatile contents of basalts and mantle xenoliths from this region are non-existent. This is a glaring omission from the geochemical data given the importance of volatiles. The focus of this study will be to examine the regional and temporal variations in volatile contents and microstructure of geochemically well-characterized Eocene-recent basalts and associated mantle xenoliths co-located with the seismic array from southern Patagonia. The lavas and their xenoliths in this region represent smaller melt volumes tapped locally from areas lacking steady-state magma chambers, experience relatively less mixing and differentiation, and offer the best opportunity to determine the composition, lithology and volatile contents of the mantle sources. The integration of these geochemical and microstructural data to a previously funded seismic experiment will enable better constraints on the temperature, composition, viscosity and density structure of the mantle beneath southern Patagonia. 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.