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

金、铂、锇、铱、钌、铑、钯和铼（统称为高度亲铁元素）是人类可获得的最稀有元素。它们在技术和艺术领域的广泛应用导致了高昂的成本。这也为大规模采矿提供了理由，而采矿对环境和人类健康的影响很大。它们稀缺的原因是，当地球分裂成硅酸盐外层（地幔和地壳）和金属地核时，它们被清除到地核中。即使这些元素在地幔中高度耗尽，先前的实验工作表明，相对于对地核清除的预期，它们的含量过高。现有的实验表明，地幔应该完全没有这些元素，但事实并非如此。对于实验和观测之间的差异，一个可能的解释是，高亲铁元素是在地核形成后，由于陨石物质的后期撞击而进入地幔的。然而，地幔和陨石的组成存在差异，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。这些实验受限于他们所能达到的压力-温度条件，并且依赖于大量的外推来推断堆芯的清除效率。一种新的实验方法依赖于选择高亲铁元素的激光电离，再加上在金刚石砧细胞中进行的实验，这些实验通常可以达到70万个大气压和4500 K的温度，这将允许在岩心形成条件下测量高亲铁元素的分配，而不依赖于大的外推。这项工作将应用一种相对较新的技术（共振电离质谱- RIMS）进行现场超痕量分析，该技术可以在地球科学以外的各种领域找到应用，包括材料科学/开发和核法医。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。pi还将通过法美科学节参与K-12级的外联活动。地幔中高亲铁元素（HSEs）消耗的原因是它们被移到地核中，随后由占地球质量约0.5%的地外物质后期增加而补充。首先，这种延迟向地球传递球粒物质的模型可以解释地幔中高热量ses的丰度，但它无法解释地幔中相对于球粒陨石和其他高热量ses的Ru/Pt和Pd/Pt比率的升高。对这些高比率的一种解释是，与其他hsse相比，Ru和Pd可能不那么亲铁或亲铜，导致它们在地核形成时部分保留在地幔中。然而，验证这一假设是困难的，因为相关的金属/硅酸盐分配实验是在P-T条件下进行的，与岩心形成期间普遍存在的条件相距甚远。研究人员将通过一种新型的超微量元素定量技术（称为RIMS）来研究地幔中HSEs的起源，该技术在金属硅酸盐实验中使用活塞缸和金刚石砧细胞（dac）来测量选定的HSEs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，研究小组将研究纳米/微金属块对金属/硅酸盐分配数据的影响，并将测量0-70 GPa和2100-4500 K的Ru、Pd和Pt的分配系数，这些条件与岩心形成有关。该奖项反映了美国国家科学基金会的法定使命，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。

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.