Temporal Evolution of Highly Siderophile Element Abundances in Earth's Mantle

点击翻译按钮获取中文摘要

Intellectual merit. The proposed study is aimed at examining the temporal evolution of the relative and absolute abundances of highly siderophile elements (HSEs: Ru, Pd, Re, Os, Ir, and Pt) in oceanic mantle and in deep mantle sources of plume-derived komatiite systems. High precision measurements of HSE concentrations and also 187Os/188Os and 186Os/188Os isotope ratios will be used to examine whether substantive changes have occurred in the HSE characteristics of the mantle sources of komatiites over a 3.4 Ga period. A total of approximately 150 chemically well-characterized Archean, Proterozoic and Phanerozoic komatiite samples and mineral separates spanning a range of ages from 3.5 Ga (Barberton and Schapenburg, S. Africa) to 89 Ma (Gorgona Island, Colombia) will be examined. Basic questions we will address include whether or not the HSE in the sources of the different komatiites are broadly chondritic, and if not, attempt to discover the reasons why they differ. Also, we will attempt to assess whether or not the absolute abundances of HSE in komatiitic sources vary either with time or location. A modicum of existing 186Os/188Os, 187Os/188Os, and HSE data for some of these komatiites already suggests that differences do exist. Consequently, we will specifically target some rocks for which there are hints of unusual mantle HSE characteristics (e.g. Belingwe, Pechenga, Gorgona), along with more typical systems, such as komatiites from Finish Lapland. Differences will be considered within a context of variable late accretion or high-pressure segregation of the core, oceanic crust recycling, melt extraction and core-mantle interaction. This project will make a substantial contribution towards building the still limited high-quality database on HSE abundances in the Earth's mantle, especially during the first 3 billion years of Earth's evolution. As such, the results of this study may allow us to ultimately place chemical constraints on the origin of late accreted materials, and possibly even lead to further evaluation of the thermal evolution of the core. Broader impacts. The study will involve the active participation of one to two undergraduate students as part of their required senior research thesis work, the capstone research project of their career at the University of Maryland. As such, some of the research proposed here will help to indoctrinate budding scientists in the methods used in petrology and isotopic geochemistry. Our outreach efforts also will include collaborations with scientists from four continents.