Boron Isotope Geochemistry of Metasomatic Minerals in Subcontinental Mantle

Arizona State University, Scottsdale AZ

Investigators

Abstract

It is at present not known if the Earth's interior is getting wetter, drier, or remaining essentially unchanged in its water content. This work will use the isotopic composition of boron to trace the past movement of water-rich fluids through Earth's mantle in an attempt to shed light on this question. The issue is important because such fluids play a key role in the evolution of Earth's surface environment, the geochemical and geophysical evolution of its interior, and the formation of economically significant ore deposits. This project will lead to an improved understanding of volatile element cycling and mantle evolution and will fulfill broader impacts related to graduate student and high- school teacher training in science, as well as international scientific collaboration. Boron (B) is a fluid-transported element whose isotopic composition can be used to trace the movement of fluids between reservoirs at Earth's surface and those in its deep interior. Using the isotopic compositions of B, Sr, Pb and Ar (measured by SIMS, LA- MC-ICPMS and LA-noble gas mass spectrometry, respectively) in minerals formed from successive generations of fluids passing through the upper mantle beneath continents, this research will trace the origin and timing of these fluids, determine their relationship to surface and deep interior geochemical reservoirs, and examine evidence for possible secular changes in the cycling of B over Earth's history. A key question to be addressed is the degree to which recycled materials giving rise to distinct mantle isotopic reservoirs are depleted in their surface-derived volatile components during subduction. By sampling and analyzing minerals formed within the mantle using in-situ microbeam methods the contaminating effects of high concentrations of B in the crust and oceans that have generated some uncertainty in previous studies may be avoided. Research will focus on samples from the Archean mantle of South Africa and younger mantle from the western US, contributing broadly to understanding the tectonic evolution of those key regions. In addition to training of a US female graduate student and field instruction of local K-12 science teachers, broader impacts of the project include collaboration with South African and European scientists that will promote international scientific exchange, catalyze development of scientific capacity, and enhance opportunities for future transdisciplinary science.