Chalcophile Element Geochemistry

亲铜元素地球化学

• 批准号: 1757313
• 负责人: Roberta Rudnick
• 金额: $ 13.87万
• 依托单位: University of California-Santa Barbara
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2018
• 资助国家: 美国
• 起止时间: 2018-02-01 至 2021-01-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1757313&HistoricalAwards=false
• 关键词: Chalcophile; Element; Geochemistry

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.

对于经济上重要的亲铜（亲硫）元素的地球化学行为，由于在分析这些元素时存在于普通岩石中的低浓度的历史困难，人们对它们的了解相对较少。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，纳克/克）下以相对高精度分析这些元素的能力。这个团队的目标是利用名义上的亲铜元素（特别是Cu、Ga、Ge、As、Mo、Ag、Cd、In、Sn、Sb、W、Tl、Pb和Bi）的浓度来解决地球科学中的三个基本问题：a)大陆地壳是如何随着时间的推移形成和演化的？b)在23 - 24亿年前发生的“大氧化事件”（GOE）之后，大气中的氧气含量是多少？c)地球的挥发性元素库存是多少？研究人员将通过三个互补的项目来调查这些问题。首先，通过分析代表古代俯冲海洋地壳残留物的岩石（榴辉岩）中钼的浓度，并通过确定产生花岗岩的岩浆分异过程中钼的行为，试图理解为什么相对于稀土元素，钼在花岗岩中被系统地耗尽。通过这些研究，他们将评估上地壳Mo的耗尽是由于其在俯冲板块中保留，还是由于其在下大陆地壳堆积中保留，还是由于其在岩浆气相中划分并沉淀在热液脉中的硫化钼（辉钼矿）中。这些结果将揭示上大陆地壳（UCC）生成的主要过程，并将限制UCC中辉钼矿的总量，从而使古代大气中pO2的计算更加可靠。第二个项目旨在确定在地质历史上上大陆地壳钼同位素组成变化的原因，正如在冰川沉积物（二晶岩）中观察到的那样。研究小组将分析几个现代风化剖面中的Mo同位素，以确定在氧气存在的情况下开始的风化是否可以解释辉晶岩的数据。如果他们发现Mo同位素在今天的含氧大气风化过程中发生了分馏，这可能使他们能够利用辉长岩数据来限制过去大气中氧气的含量。最后，他们将确定在比利牛斯山脉暴露的上地幔岩石中全套亲铜元素的分布。不像类似的岩石在岩浆中被迅速运送到地球表面，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中高挥发性亲铜元素（如as， Cd, Ga, in， Sn和Tl）的丰度。确定地球的挥发性元素丰度有助于了解我们的星球是如何形成的。

项目成果

Geochemistry of molybdenum in the continental crust

• DOI: 10.1016/j.gca.2018.06.039
• 发表时间: 2018-10-01
• 期刊: GEOCHIMICA ET COSMOCHIMICA ACTA
• 影响因子: 5
• 作者: Greaney, Allison T.;Rudnick, Roberta L.;Clemens, John D.
• 通讯作者: Clemens, John D.

Molybdenum isotope fractionation in glacial diamictites tracks the onset of oxidative weathering of the continental crust

• DOI: 10.1016/j.epsl.2020.116083
• 发表时间: 2020-03-15
• 期刊: EARTH AND PLANETARY SCIENCE LETTERS
• 影响因子: 5.3
• 作者: Greaney, Allison T.;Rudnick, Roberta L.;Anbar, Ariel D.
• 通讯作者: Anbar, Ariel D.

Assessing molybdenum isotope fractionation during continental weathering as recorded by weathering profiles in saprolites and bauxites

• DOI: 10.1016/j.chemgeo.2021.120103
• 发表时间: 2021-04
• 期刊: Chemical Geology
• 影响因子: 3.9
• 作者: A. Greaney;R. Rudnick;S. Romaniello;Aleisha C. Johnson;A. Anbar;M. Cummings