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.

对经济上重要的亲铜（喜硫）元素的地球化学行为的了解相对较少，因为历史上分析这些元素在常见岩石中的低浓度时存在困难。分析方法的最新进展现在提供了在非常低（例如纳克每克）浓度水平下以相对较高的精度分析这些元素的能力。该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）大陆地壳如何随着时间的推移而形成和演化？b）在发生“大氧化事件”（GOE）之后，大气中的氧气含量是多少？ 2.3-24亿年前？以及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。第一个试图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，并确定钼在产生花岗岩的岩浆分异过程中的行为，来了解为什么花岗岩中的钼相对于稀土元素会系统性地贫化。通过这些研究，他们将评估上地壳钼的贫乏是否是由于其在俯冲板片中的滞留、在下陆壳堆积物中的滞留，或其在热液脉中划分为岩浆气相和硫化钼（辉钼矿）中的沉淀所致。研究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，从而可以更可靠地计算古代大气中的 pO2。第二个项目旨在确定地质历史上上大陆地壳钼同位素组成变化的原因，如在冰川沉积物（混积岩）中观察到的那样。该团队将分析几种现代风化剖面中的钼同位素，以确定在氧气存在下风化的开始是否可以解释混积岩数据。如果他们发现钼同位素在当今含氧大气的风化过程中发生分馏，那么他们可能可以使用混积岩数据来限制过去存在的大气氧含量。最后，他们将确定现在暴露在比利牛斯山脉的上地幔岩石中全套亲铜元素的分布。与在岩浆中迅速迁移到地球表面的类似岩石不同，这些地幔露头保留了硫化物，因此将允许研究人员确定这些元素在地幔熔化过程中的行为，并且还可以对地球中中等至高挥发性亲铜元素（如砷、镉、镓、铟、锡和铊）的丰度进行独立估计。确定地球的挥发性元素丰度可以深入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