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

金、铂、铯、铱、钚、铑、钯和Re(统称为高度亲铁元素)是人类可获得的最稀有元素。它们在技术和艺术中的广泛使用导致了高昂的成本。这也证明了广泛的采矿是合理的，因为这对环境和人类健康有很大的影响。它们稀少的原因是，当地球分裂成硅酸盐外层(地幔和地壳)和金属核时，它们被清除到地核。即使这些元素在地幔中高度贫化，先前的实验工作也表明，与地核清除的预期相比，它们是过剩的。现有的实验表明，地幔中应该完全没有这些元素，而这并不是我们所看到的。实验和观测之间的这种差异的一个可能的解释是，在地核形成后，由于陨石物质的晚期撞击添加，高度亲铁的元素被输送到地幔。然而，地幔和陨石的成分之间存在差异，特别是对于Ru，一个重要的问题是，以前的实验是否可靠地预测了地核中高度亲铁元素的清除。这些实验受限于它们所能达到的压力-温度条件，并依赖于大量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元素的激光电离，再加上在钻石顶压室中进行的实验，这些实验通常可以达到700,000个大气压和4500 K的温度，这将允许在岩心形成条件下测量高度亲铁元素的分配，而不需要依靠大量的外推。这项工作将把一种相对较新的技术(共振电离质谱学-RIMS)应用于原位超痕量分析，该技术可在地球科学以外的各种领域找到应用，包括材料科学/开发和核取证。该项目是地球化学家、物理学家和仪器开发商、一名实验岩石学家和一名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。PIS还将通过法美科学节参与K-12级别的外联活动。地幔中高度亲铁元素(HSE)耗尽的原因是它们移入地球核心，随后通过占地球质量约0.5%的外星人物质的后期吸积而补充。首先，这种延迟向地球输送球粒陨石物质的模型可以解释地幔中HSE的丰度，但它不能解释地幔中相对于球粒陨石和其他HSE的Ru/Pt和Pd/Pt比值的升高。这些高比率的一种解释是，与其他HSE相比，Ru和Pd可能不那么亲铁或亲硫，导致它们在核心形成时部分保留在地幔中。然而，验证这一假说是困难的，因为相关的金属/硅酸盐分配实验是在P-T条件下进行的，而P-T条件与人们认为的在核心形成期间占优势的条件相去甚远。研究人员将通过应用一种名为RIMS的新的超痕量元素量化技术来研究地球地幔中HSE的来源，该技术在使用活塞柱和钻石砧座(DAC)进行的金属硅酸盐实验中测量选定的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的合作，研究组将研究纳米/微型金属块对金属/硅酸盐分配数据的影响，并将在0-70 GPA和2100-4500K测量Ru、Pd和Pt的分配系数，跨越与岩芯形成相关的条件。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.