Collaborative Research: The Lu-Hf Composition of the Earth and Implications for the Future of Hf Isotope Geochemistry

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This research project is directed at solving pressing problems that currently hinder the application of the Lu-Hf isotopic system to mantle and crust evolution. The value of the decay constant of 176Lu is known to no better than 6%, and the Lu-Hf composition of the Bulk Silicate Earth (BSE), essential for studies of Earth differentiation, could vary by around 10% in Lu/Hf ratio, leading to vastly different interpretations of Hf isotopic data for old rocks. It is proposed to conduct Lu- Hf isotopic analyses of (1) phosphate mineral separates from chondrite meteorites, to constrain the 176Lu decay constant and (2) powders of chondrites prepared from 6- to 30-gram masses of meteorite, to constrain BSE. This project will be done in collaboration with Christa Gopel (Institut de Physique du Globe de Paris), who has extensive experience separating phosphates from chondrites. She will supervise and conduct mineral separations using powders of large volumes of chondrites made 20 years ago that are available from the Smithsonian Institution. Chemical and mass spectrometric work will be split between Arizona and Washington State, with no resulting increased cost to NSF. In terms of broader impacts, this work will contribute materially to training of professional isotope geochemists at a postdoctoral level. In addition, one or more undergraduate students at both institutions will learn elements of laboratory procedure and approaches to research.