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

地核、地幔、地壳元素分配研究

• 批准号: 0739006
• 负责人: William McDonough
• 金额: $ 31.66万
• 依托单位: University of Maryland, College Park
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-04-01 至 2013-03-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0739006&HistoricalAwards=false
• 关键词: Studies; Partitioning; Elements; Between; Core

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.

智力上的优点。 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进一步了解，将大大提高我们对地核和周围硅酸盐壳的组成的理解，特别是关于产热元素U、Th和K的丰度和分布。这些见解对于理解驱动地球发电机、地幔对流和板块构造的能源，以及理解地球的挥发性元素存量和限制核幔交换的性质和程度至关重要。该提案要求提供资金，用于分析天然和合成样品的成分，重点是精确和准确地确定亲铁、亲铜和关键亲石元素的微量丰度，以确定金属（以及在较小程度上的硫化物）和硅酸盐体系的固-液、固-固和液-液分配系数。这些数据将用于改进对地核、地幔、地壳和块状硅酸盐地球的整体成分的估计，并提供对地核形成、壳幔演化和地核地幔交换过程中元素分配的定量见解。（1）地球储层中亲铁元素和某些亲铜元素的富集和行为（核-幔-壳）和岩浆，（2）名义上亲石元素的丰度和行为（例如，Nb、Ta、K、Th和U）;（3）地幔和地核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中的丰度和分布的理解，（B）产生这些元素在核-幔和幔-壳分异过程中的分布系数，（c）限制核分离和核-幔交换已经发生和/或正在发生的条件，以及（d）更好地定义行星尤里数（辐射热产生与总热损失的比率）。 我的小组以前的研究中最重要的发现（在本提案中描述）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）。我们的MORB的K/U值（19，800 +/- 3400，n=43）与Jochum等人（1983）报道的值有显著差异，反映了抽样和群体方差处理的差异。现代地幔的K/U值估计高于大陆地壳，可能包含硅酸盐地球50%的K和U预算，表明40 K的放射性贡献更大。(2)现代玄武岩具有相对恒定的W/Ba（0.00136 +/- 86，2sd），这与大陆地壳相当，并推断代表硅酸盐地球。由此，我们可以确定硅酸盐地球的W浓度（9.0 +/- 5.8 ppb W）。使用这个修正值，我们测试了基于Os同位素为夏威夷苦橄岩提出的核-幔交换假设，并发现如果这些熔岩来自具有3x硅酸盐地球W含量（~27 ppb）的储层和含有516 ppb W的核（即，约占地球W预算的96%）。(3)熔融金属和硅酸盐熔体之间的新实验分配数据（Corgne等人）与氧逸度增加（最初接近IW-3，最后接近IW-2）下的核心形成一致。我们的研究结果是一致的核心包含一小部分的Zn，Ga，K，Cu，Mn，Cr，Nb和Ta的行星预算。一个超临界Nb/Ta比已被提出的核心，但我们的数据的上限在15 ppb的Ta。我们的模型预测一个核心，至少有10 ppm的钛。此外，核心预计包含10%的K行星预算，但这一估计可能接近1%或更少。更广泛的影响。学生参与（在研究生，本科和高中水平）是我在UMD研究计划的一个基本方面。学生参与实践研究，这提供了重要的教育和培训机会，学生可以将其转化为学术界，工业界或政府的未来工作。该项目将支持一名博士生和两名本科生研究人员。我在教育、服务和推广方面的记录远远超出了科学界，进入了公共领域（K-12讲座，马里兰州开放日实验室图尔斯参观和分析，高中实习生在我的质谱实验室进行为期一年的研究项目），并在本提案的正文中进行了全面描述。