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

知识价值。本研究旨在探讨海洋地幔和地幔深部地幔源中高亲铁元素（HSEs: Ru、Pd、Re、Os、Ir和Pt）的相对丰度和绝对丰度的时间演化。高精度的HSE浓度测量以及187Os/188Os和186Os/188Os同位素比值将用于检验3.4 Ga时期科马地岩地幔源的HSE特征是否发生了实质性变化。本文将对大约150个化学特征良好的太古宙、元古宙和显生宙科马地岩样品和矿物分离物进行研究，这些样品的年龄范围从3.5 Ga（南非巴伯顿和沙彭堡）到89 Ma（哥伦比亚戈尔戈纳岛）。我们将解决的基本问题包括，不同科马陨石来源中的HSE是否具有广泛的诊断性，如果不是，则试图发现它们不同的原因。此外，我们将尝试评估科马提岩源中HSE的绝对丰度是否随时间或地点而变化。现有的少量1860o / 188o、1870o / 188o以及其中一些科马陨石的HSE数据表明，差异确实存在。因此，我们将专门针对一些具有不寻常地幔HSE特征的岩石（例如Belingwe， Pechenga, Gorgona），以及更典型的系统，例如来自Finish Lapland的komatiites。差异将在可变的晚期增生或核心高压分离、海洋地壳循环、熔体提取和核幔相互作用的背景下考虑。该项目将为建立目前仍有限的、关于地幔中HSE丰度的高质量数据库做出重大贡献，特别是在地球演化的前30亿年。因此，这项研究的结果可能使我们最终能够对晚期吸积物质的起源施加化学限制，甚至可能导致对地核热演化的进一步评估。更广泛的影响。这项研究将要求一到两名本科生积极参与，作为他们要求的高级研究论文工作的一部分，这是他们在马里兰大学职业生涯的顶点研究项目。因此，这里提出的一些研究将有助于向崭露头角的科学家灌输岩石学和同位素地球化学中使用的方法。我们的外联工作还将包括与来自四大洲的科学家合作。

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.