CSEDI: Collaborative Research: Experimental Partitioning of Highly Siderophile Elements at Ultratrace Level for Understanding the Conditions of Core Formation

CSEDI：合作研究：超痕量水平的高亲铁元素的实验分配，以了解核心形成的条件

• 批准号: 2001043
• 负责人: Michael Krawczynski
• 金额: $ 13.67万
• 依托单位: Washington University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2020
• 资助国家: 美国
• 起止时间: 2020-05-01 至 2025-04-30
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2001043&HistoricalAwards=false
• 关键词: CSEDI; Collaborative; Research; Experimental; Partitioning

Gold, platinum, osmium, irridium, ruthenium, rhodium, palladium, and rhenium (known collectively as highly siderophile elements) are among the rarest elements available to mankind. Their widespread use in technology and the arts results in a high cost. It also justifies extensive mining, which has a high environmental and human health impact. The reason for their scarcity is that they were scavenged into the core when Earth separated into a silicate outer layer (mantle and crust) and a metallic core. Even if these elements are highly depleted in the mantle, previous experimental work indicates that they are overabundant relative to expectation for scavenging by the core. Available experiments suggest that the mantle should be completely barren of these elements, which is not what is seen. A likely explanation for this discrepancy between experiments and observations is that the highly siderophile elements were delivered into the Earth's mantle by the late impact addition of meteoritic material after the core had formed. There are differences however between the composition of the mantle and meteorites, notably for ruthenium, and an important question is whether previous experiments reliably predict the scavenging of highly siderophile elements in the core. These experiments were limited in the pressure-temperature conditions that they could achieve and relies on large extrapolations to make inferences about the scavenging efficiency of the core. A new experimental approach relying on the laser ionization of selected highly siderophile elements coupled with experiments done in diamond anvil cells that can routinely reach pressures of 700,000 atmospheres and temperatures of 4500 K, will allow the partitioning of highly siderophile elements to be measured under core formation conditions without relying on large extrapolations. This work will apply a relatively new technique (Resonant Ionization Mass Spectrometry - RIMS) to in situ ultratrace analyses, which can find applications in a variety of fields outside of Earth sciences, including material sciences/development and nuclear forensics. The project is a multidisciplinary collaboration between geochemists, physicists and instrument developers, an experimental petrologist, and a high-pressure mineral physicist. The project will support two graduate as well as undergraduate students, who will be trained on a multidisciplinary research project. The PIs will also be involved in outreach at the K-12 level through the French-American Science Festival.The reason for the depletions in highly siderophile elements (HSEs) in the mantle is their removal into Earth’s core, and their subsequent replenishment by late accretion of extraterrestrial material representing ~0.5 % of Earth’s mass. To first order, this model of late delivery of chondritic material to the Earth can account for the abundance of HSEs in the mantle but it fails to explain the elevated Ru/Pt and Pd/Pt ratios in the mantle relative to chondrites and other HSEs. One explanation for these high ratios is that Ru and Pd may be less siderophile or chalcophile compared to other HSEs, resulting in their partial retention in mantle when the core formed. Testing this hypothesis is however difficult because the relevant metal/silicate partitioning experiments have been done at P-T conditions that are quite remote from those that are thought to have prevailed during core formation. The investigators will study the origin of HSEs in Earth’s mantle by applying a novel ultra-trace element quantification technique known as RIMS to measure the concentrations of selected HSEs in metal-silicate experiments done using piston cylinders and diamond anvil cells (DACs). Through this collaboration between geochemists, physicists and instrument developers, an experimental petrologist, and a high-pressure mineral physicist, the research group will study the effect of nano/micro metal nuggets on metal/silicate partition data, and will measure the partition coefficients of Ru, Pd, and Pt at 0–70 GPa and 2100–4500 K, which spans conditions relevant to core formation.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

金、铂、锇、铱、钌、铑、钯和铱（统称为高亲铁性元素）是人类可获得的最稀有的元素之一。它们在技术和艺术中的广泛使用导致了高昂的成本。这也证明了大规模采矿是有道理的，因为大规模采矿对环境和人类健康有很大影响。它们稀少的原因是，当地球分离成硅酸盐外层（地幔和地壳）和金属内核时，它们被清除到内核中。即使这些元素在地幔中高度贫化，以前的实验工作表明，它们相对于被地核清除的预期是过剩的。现有的实验表明，地幔应该是完全贫瘠的这些元素，这是不是什么看到的。实验和观测之间这种差异的一个可能的解释是，高度亲铁的元素是在地核形成后，通过后期撞击加入的陨石物质进入地幔的。然而，地幔和陨石的成分之间存在差异，特别是钌，一个重要的问题是以前的实验是否可靠地预测了核心中高度亲铁元素的清除。这些实验局限于它们能够达到的压力-温度条件，并且依赖于大的外推来推断核心的清除效率。一种新的实验方法依赖于选定的高度亲铁性元素的激光电离，再加上在金刚石砧室中进行的实验，通常可以达到700，000个大气压和4500 K的温度，将允许在核心形成条件下测量高度亲铁性元素的分区，而不依赖于大的外推。这项工作将把一种相对较新的技术（共振电离质谱法）应用于现场超痕量分析，这种技术可以应用于地球科学以外的各种领域，包括材料科学/发展和核法医学。 该项目是地球化学家，物理学家和仪器开发人员，实验岩石学家和高压矿物物理学家之间的多学科合作。该项目将支持两名研究生和本科生，他们将接受多学科研究项目的培训。研究所还将通过法国-美国科学节参与K-12级的外联活动。地幔中高度亲铁元素（HSE）耗尽的原因是它们被移到地核中，随后通过占地球质量约0.5%的地外物质的晚期吸积来补充。一阶，这个模型的晚交付的南极物质到地球可以解释的丰富的HSEs在地幔中，但它不能解释的Ru/Pt和Pd/Pt比值升高的地幔相对于南极和其他HSEs。这些高比率的一个解释是，Ru和Pd可能是较少的亲铁性或亲铜性相比，其他HSE，导致他们的部分保留在地幔时，核心形成。然而，检验这一假设是困难的，因为相关的金属/硅酸盐分配实验是在P-T条件下进行的，这些条件与被认为在岩心形成期间占主导地位的条件相距甚远。研究人员将通过应用一种称为RIMS的新型超痕量元素定量技术来研究地球地幔中HSE的起源，以测量使用活塞缸和金刚石砧室（DAC）进行的金属硅酸盐实验中选定HSE的浓度。通过地球化学家，物理学家和仪器开发人员，实验岩石学家和高压矿物物理学家之间的合作，研究小组将研究纳米/微米金属块对金属/硅酸盐分配数据的影响，并将测量Ru，Pd和Pt在0-70 GPa和2100-4500 K的分配系数，该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。