Chalcophile Element Geochemistry

经济上重要的亲硫(亲硫)元素的地球化学行为相对较少，因为历史上很难在普通岩石中出现的低浓度下分析这些元素。分析方法的最新进展现在提供了在非常低的浓度水平(例如每克纳克)以相对较高的精度分析这些元素的能力。这个团队的目标是利用名义上亲硫元素的浓度(特别是铜、镓、锗、砷、钼、银、镉、铟、锡、锑、钨、铊、铅和铋)来解决地球科学中的三个基本问题：a)大陆地壳是如何形成和随时间演变的？b)发生在23-24亿年前的“大氧化事件”(GOE)后，大气中有多少氧气？c)地球上的挥发性元素库存是什么？研究人员将通过三个互补的项目来研究这些问题。第一种是通过分析钼在代表古代俯冲洋壳残留物的岩石(榴辉岩)中的浓度，以及通过确定钼在产生花岗岩的岩浆分异过程中的行为，试图理解为什么相对于稀土元素，花岗岩中的钼被系统地贫化。通过这些研究，他们将评估上地壳钼的贫化是由于它滞留在俯冲板块中，还是由于它滞留在下陆壳堆积体中，还是由于它被分成岩浆气相并在热液脉体中以硫化钼(辉钼矿)的形式沉淀。这一结果将揭示负责形成上大陆地壳(UCC)的主要过程，并将限制UCC中辉钼矿的总量，从而能够更可靠地计算古代大气中的pO2。第二个项目旨在确定在冰川沉积(硅质岩)中观察到的陆壳上部钼同位素组成在地质历史上发生变化的原因。该团队将分析几个现代风化剖面中的钼同位素，以确定在氧气存在的情况下开始风化是否可以解释英云母数据。如果他们发现在今天富氧的大气中，钼同位素在风化过程中发生了分馏，这可能会使他们能够利用透辉石数据来限制过去存在的大气氧气的数量。最后，他们将确定全套亲硫元素在比利牛斯山脉暴露的上地幔岩石中的分布。与在岩浆中迅速运往地球表面的类似岩石不同，这些地幔露头保留了硫化物，因此将使研究人员能够确定这些元素在地幔融化期间的行为，并允许独立估计地球上中等到高度挥发性亲磷元素的丰度，如As、Cd、Ga、In、Sn和Tl。确定地球的挥发性元素丰度有助于深入了解我们的星球是如何形成的。

The geochemical behavior of the economically important chalcophile (sulfur-loving) elements is relatively poorly understood due to historical difficulties in analyzing these elements at the low concentrations in which they occur in common rocks. Recent advances in analytical methods now provides the ability to analyze these elements at relatively high precision at very low (e.g., nanogram per gram) concentration levels. This team aims to use the concentrations of nominally chalcophile elements (specifically Cu, Ga, Ge, As, Mo, Ag, Cd, In, Sn, Sb, W, Tl, Pb, and Bi) to address three fundamental questions in Earth science: a) how has the continental crust formed and evolved over time?, b) how much oxygen was in the atmosphere following the "Great Oxidation Event" (GOE) that occurred at 2.3-2.4 billion years ago?, and c) what is Earth's volatile element inventory?Researchers will investigate these questions through three complementary projects. The first seeks to understand why molybdenum is systematically depleted in granites relative to rare earth elements by analyzing its concentrations in rocks (eclogites) that represent the residues of ancient subducted oceanic crust, and by determining the behavior of Mo during magmatic differentiation that produces granites. Through these studies, they will evaluate whether the upper crustal Mo depletion is due to its retention in subducted slabs, its retention in lower continental crustal cumulates, or its partitioning into a magmatic vapor phase and precipitation in molybdenum sulfide (molybdenite) in hydrothermal veins. The results will shed light on the main processes responsible for generation of the upper continental crust (UCC) and will place constraints on total amount of molybdenite in the UCC, allowing for more robust calculation of pO2 in the ancient atmosphere. The second project seeks to determine the cause of the change in the molybdenum isotope composition of the upper continental crust over geologic history, as observed in glacial deposits (diamictites). The team will analyze Mo isotopes in several modern weathering profiles to determine whether the onset of weathering in the presence of oxygen can explain the diamictite data. If they find that Mo isotopes fractionate during weathering in today's oxygenated atmosphere, it may allow them to use the diamictite data to place constraints on the amount of atmospheric oxygen that was present in the past. Finally, they will determine the distribution of the full suite of chalcophile elements within rocks of the upper mantle that are now exposed in the Pyrenees Mountains. Unlike similar rocks that are rapidly transported to Earth's surface in magmas, these mantle outcrops have retained sulfides, and thus will allow researchers to determine how these elements behave during mantle melting, and also allow for an independent estimate of Earth's abundance of moderately to highly volatile chalcophile elements such as As, Cd, Ga, In, Sn and Tl. Determining the volatile element abundances of Earth provides insights into how our planet formed.