Studies on the Partitioning of Elements Between the Core, Mantle and Crust

知识价值。提高对地球形成和随后的地核形成过程中普遍存在的压力、温度和气体逸度条件的认识，将大大提高我们对地核和周围硅酸盐壳的组成的理解，特别是对产生热量的元素U、Th和k的丰度和分布的理解。这些见解对于理解驱动地球动力学的能量来源至关重要。地幔对流和板块构造，以及了解地球的挥发性元素库存和限制核幔交换的性质和程度。该提案要求资金用于分析天然和合成样品的组成，重点是精确和准确地测定亲铁、亲铜和关键亲石元素的痕量丰度，以便建立金属（以及较小程度的硫化物）和硅酸盐系统的固-液、固-固和液-液分布系数。这些数据将用于改进地核、地幔、地壳和硅酸盐大地的整体组成的估计，并为地核形成、壳幔演化和核幔交换过程中的元素分配提供定量的见解。本研究的主要目标包括：(1)确定地球储集层（核-幔-壳）和岩浆中亲铁元素和部分亲铜元素的浓度和行为；(2)确定名义亲石元素（如Nb、Ta、K、Th和U）在岩心形成过程中的丰度和行为；(3)确定地幔和岩心中挥发性元素的收支。这些相互关联的项目的结果将(a)提高我们对这些元素在地球上的丰度和分布的理解，(b)在核-幔和幔-壳分化期间这些元素的产率分布系数，(c)约束核分离和核-幔交换发生和/或正在发生的条件，以及(d)更好地定义行星Urey数（放射性产热与总热损失的比值）。我的团队之前的调查（在本提案中描述）中最重要的发现包括：(1)来自洋脊，岛屿和弧的现代玄武岩的加权平均K/U为20,600+/-3600 （2sd,n = 50）。我们对MORB的K/U值（19,800 +/- 3400,n=43）与Jochum等人（1983）的报告有显著差异，反映了抽样和处理总体方差的差异。据估计，现代地幔的K/U值高于大陆地壳，可能包含硅酸盐地球的50%的K和U收支，表明40K的放射性贡献更大。(2)现代玄武岩具有相对稳定的W/Ba (0.00136 +/- 86,2 sd)，与大陆地壳相当，可以推断为硅酸盐地球。由此，我们可以确定硅酸盐土的W浓度（9.0 +/- 5.8 ppb W）。利用这一修正值，我们测试了基于Os同位素提出的夏威夷苦荞岩的核幔交换假设，发现如果这些熔岩来自含3倍硅酸盐地球W含量（~27 ppb）和含516 ppb W的岩心（即~96%的地球W收支）的储层，则岩心成分的合并与W同位素数据一致。(3)新的金属熔体与硅酸盐熔体的实验划分数据（Corgne等）与氧逸度增加时岩心形成（开始于IW-3附近，结束于IW-2附近）相一致。我们的结果与含有Zn， Ga， K, Cu, Mn, Cr， Nb和Ta的一小部分行星预算的核心一致。超球粒Nb/Ta比值已被提出，但我们的数据表明其上限为~15 ppb Ta。我们的模型预测一个核心至少有10ppm的钛。此外，据预测，地核包含地球预算K的10%，但这一估计可能更接近1%或更少。更广泛的影响。学生参与（研究生、本科生和高中阶段）是我在马里兰大学的研究项目的一个基本方面。学生参与动手研究，这提供了重要的教育和培训机会，学生可以转化为未来在学术界，工业界或政府工作。本项目资助博士生1名，本科生2名。我在教育、服务和拓展方面的记录远远超出了科学界，进入了公共领域（K-12讲座、马里兰日开放日实验室参观和分析、在我的质谱实验室进行为期一年的研究项目的高中实习生），这些都在本提案的主体中得到了完整的描述。

Intellectual merit. Improved knowledge of the pressure, temperature and gas fugacity conditions prevalent during the formation of the Earth and subsequent core formation would substantially enhance to our understanding of the composition of the Earth's core and surrounding silicate shell, particularly with respect to the abundance and distribution of the heat-producing elements U, Th and K. Such insights are essential for understanding the energy sources that drive the geodynamo, mantle convection and plate tectonics, as well as understanding the volatile element inventory of the planet and constraining the nature and degree of core-mantle exchange. This proposal requests funds to analyze the compositions of natural and synthetic samples, with emphasis on precise and accurate determinations of trace abundances of siderophile, chalcophile and key lithophile elements in order to establish solid-liquid, solid-solid and liquid-liquid distribution coefficients for metallic (and to a lesser extent sulfidic) and silicate systems. These data will be used to develop improved estimates of bulk compositional of the core, mantle, crust and bulk silicate Earth, and to provide quantitative insights into the partitioning of elements during core formation, crust-mantle evolution, and core-mantle exchange.The main goals of the proposed study include determining: (1) the concentration and behavior of siderophile and some chalcophile elements in Earth reservoirs (core-mantle-crust) and magmas, (2) the abundance and behavior of nominally lithophile elements (e.g., Nb, Ta, K, Th, and U) during core formation, and (3) the budget of volatile elements in the mantle and core. Results from these interrelated projects will (a) improve our understanding of the abundance and distribution of these elements in the Earth, (b) yield distribution coefficients for these elements during core-mantle and mantle-crust differentiation, (c) constrain the conditions under which core separation and core-mantle exchange has occurred and/or is occurring, and (d) better define the planetary Urey number (ratio of radiogenic heat production to total heat loss). The most important findings of my group's previous investigations (described in this proposal) include: (1) Modern basalts from ocean ridges, islands and arcs have a weighted average K/U of 20,600+/-3600 (2sd,n = 50). Our K/U value for MORB (19,800 +/- 3400, n=43) differs significantly from that reported by Jochum et al. (1983), reflecting differences in sampling and treatment of the population variance. The modern mantle is estimated to have a K/U value higher than the continental crust and likely contains 50% of the K and U budget of the silicate Earth, indicating a larger radioactive contribution of 40K. (2) Modern basalts have relatively constant W/Ba (0.00136 +/- 86, 2sd), which is comparable to the continental crust and by inference represents the silicate Earth. From this, we can establish the W concentration of the silicate Earth (9.0 +/- 5.8 ppb W). Using this revised value we tested the hypothesis of core-mantle exchange proposed for Hawaiian picrites based on Os isotopes and found that the incorporation of a core component is consistent with W-isotope data, if these lavas are derived from a reservoir with 3x silicate Earth W content (~27 ppb) and a core containing 516 ppb W (i.e., ~96% of the Earth's W budget). (3) New experimental partitioning data (Corgne et al) between molten metal and silicate melt are consistent with core formation under increasing oxygen fugacity (initially near IW-3 and finishing near IW-2). Our results are consistent with a core containing a fraction of the planetary budget of Zn, Ga, K, Cu, Mn, Cr, Nb and Ta. A superchondritic Nb/Ta ratio has been proposed for the core, but our data place the upper limit at ~15 ppb Ta. Our model predicts a core with at least 10 ppm Ti. Moreover, the core is predicted to contain 10% of the planetary budget of K, but this estimate is likely closer to 1% or less. Broader impacts. Student participation (at the graduate, undergraduate and high school levels) is a fundamental aspect of my research program at UMD. Students participate in hands-on research, which provides important educational and training opportunities that the students can translate to future work in academia, industry or government. This project will support one PhD student and two undergraduate researchers. My record in education, service and outreach extends well beyond the scientific community and into the public realm (K-12 lectures, Maryland Day open house lab tours and analyses, high school interns on year-long research programs in my mass spectrometry laboratory) and is described, in full, in the body of this proposal.