Fractionation of Rare-Earth and High-Field Strength Elements by Melting of a Re-equilibrated Mantle

Brown University, Providence RI

Investigators

Abstract

The abundance and distribution of rare earth elements (REE) and high field strength elements (HFSE) in mafic and ultramafic rocks provide important clues to understanding the origin and evolution of igneous rocks. A fundamental observation in geochemistry is that there is relative depletion of HFSE compared to REE and other trace elements such as thorium in basaltic magmas erupted along continental margins. Geochemists have used this observation to unravel magmatic processes in the subduction zones. However, the mechanisms leading to this relative depletion are still poorly understood. This project is designed to test if the relative depletion of HFSE in arc magmas is caused by redistribution of REE and HFSE between mantle minerals during subsolidus re-equilibration and subsequent partial melting in the subduction zone. The project would advance STEM education by providing research opportunities for undergraduate students and serving as a training ground for a graduate student. Disequilibrium melting happens when the kinetics of chemical exchange between a residual mineral and partial melt is sluggish compare to the rate of melting. It is more likely to take place during hydrous melting because the solidus temperature is lower. The primary objective of this proposed study is to assess and quantify the roles of subsolidus re-equilibration and subsequent disequilibrium partial melting of spinel lherzolite in the fractionation of HFSE from REE during magma genesis. The project has two closely related components: (1) experimental calibration of parameterized lattice strain models for the partitioning of HFSE between pyroxene and basaltic melt and between orthopyroxene and clinopyroxene, and (2) numerical and theoretical studies of HFSE and REE fractionation during disequilibrium mantle melting. Results from the proposed pyroxene-melt partitioning experiments will also be used to update existing partitioning models for REE and HFSE in pyroxenes. Temperature and composition dependent partitioning models allows the investigators to use self-consistent initial conditions for HFSE and REE abundances in starting pyroxenes in their melting model, and to conduct simulations along a melting path where temperature and mineral major element compositions change systematically. Forward and inverse geochemical modeling will be conducted to critically assess the geochemical consequences of melting of subsolidusly re-equilibrated mantle.