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

本研究项目旨在解决目前阻碍Lu-Hf同位素系统应用于地幔和地壳演化的紧迫问题。已知176Lu的衰变常数不超过6%，而对地球分异研究至关重要的大块硅酸盐土（BSE）的Lu-Hf组成在Lu/Hf比值上可能相差10%左右，导致对古老岩石Hf同位素数据的解释存在巨大差异。建议对：(1)从球粒陨石中分离的磷矿物进行Lu- Hf同位素分析，以约束176Lu衰变常数；(2)从6 ~ 30g质量的陨石中制备的球粒陨石粉末进行Lu- Hf同位素分析，以约束BSE。该项目将与Christa Gopel（巴黎环球物理研究所）合作完成，他在从球粒陨石中分离磷酸盐方面拥有丰富的经验。她将监督和指导矿物分离，使用20年前制成的大量球粒陨石粉末，这些粉末可以从史密森学会获得。化学和质谱工作将在亚利桑那州和华盛顿州分开进行，这不会增加NSF的成本。就更广泛的影响而言，这项工作将对培养博士后水平的专业同位素地球化学家作出重大贡献。此外，两所院校的一名或多名本科生将学习实验室程序和研究方法的要素。

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.