Collaborative Research: CSEDI -Understanding Si and Fe differentiation in Earth?s mantle and core through experimental and theoretical research in geochemistry and mineral physics

合作研究：CSEDI - 通过地球化学和矿物物理的实验和理论研究了解地幔和地核中的硅和铁分异

• 批准号: 1503084
• 负责人: Renata Wentzcovitch
• 金额: $ 21.5万
• 依托单位: University of Minnesota-Twin Cities
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2015
• 资助国家: 美国
• 起止时间: 2015-04-15 至 2019-03-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1503084&HistoricalAwards=false
• 关键词: Collaborative; Research; CSEDI; Understanding; Si

To first order, the Earth is divided into three concentric shells of different nature: the metallic core, the rocky mantle, and the fluid atmosphere/hydrosphere. While samples are available from the mantle and atmosphere/hydrosphere, the nature and composition of the core remain poorly understood. In particular, the core is known to be less dense than pure iron-nickel alloy, indicating that another light element is present in the core, possibly oxygen, silicium, or sulfur. The conditions (pressure and temperature) under which Earth's core formed and the nature of the light element in Earth's core are two major unresolved questions in planetary sciences. Because no core samples are directly available for study, scientists rely on remote seismic observations or other indirect methods to address those questions. In the proposed work, the approaches of geochemistry (the chemistry of the Earth), mineral physics (solid state physics applied to natural materials), and computational techniques will be combined to set limits on the temperature condition during core formation and the nature of the light element in Earth's core. This will be achieved by examining the extent to which different isotopic flavors of silicon and iron were partitioned between metal and silicate when the core formed. The work will involve synthesizing minerals in the laboratory and compressing them to pressure conditions relevant to the deep Earth by confining the samples between two diamonds, measuring the strength of the iron bonds in those minerals at a synchrotron source that produces very energetic X-rays, and examining, through computer calculations, the behavior of matter under high pressure and temperature. This work can impact many fields of science, ranging from the origin of Earth's dynamo to characterization of extrasolar planets through measurement of their mass. All PIs will actively engage in training and educating graduate students, undergraduate students, and postdocs in the proposed research projects. The PIs will continue developing SciPhon, a user-friendly, free software for NRIXS data reduction. This program will be made available to various communities studying different aspects of NRIXS, including geochemistry, mineral physics, material sciences, condensed matter physics, and biochemistry. All PIs will be actively involved in outreach programs including the UTeach Outreach Program that conducts academic summer camps for underrepresented K-12 kids from the Austin and southwest Texas area. The mass of the Earth and its accretion history are such that core-mantle differentiation was probably unavoidable but considerable uncertainties remain as to how and when this took place. Our limited understanding of this major event arises from our lack of sampling of Earth's deep interior. Scientists have devised indirect approaches to address this shortcoming by relying on (1) mineral physics experiments to reproduce the high pressure-temperature conditions prevailing in Earth's interior, (2) theoretical calculations to mimic those same conditions, and (3) geochemical measurements of the composition of mantle rocks to search for telltale signatures of core formation. These strongly interweaved approaches have led to significant progress but first-order unanswered questions remain, such as under what pressure-temperature conditions did the core form, what is the nature of the light element in the core, and did core formation fractionate Si and Fe isotopes. Terrestrial basalts have non-chondritic Si and Fe isotopic compositions, which could reflect partitioning of these elements into the core, although other interpretations exist. The investigators propose to establish Si and Fe isotope fractionation factors using high-pressure nuclear resonant inelastic X-ray scattering (NRIXS) and theoretical calculations at deep mantle conditions via collaborative approaches in geochemistry (Dauphas), theoretical ab initio calculations (Wentzcovitch), and experimental mineral physics (Lin). The derived force constants of Si and Fe bonds in basaltic glasses, lower-mantle minerals (bridgmanite and ferropericlase), and Fe alloys will allow us to build a deep-Earth geochemical model to evaluate if the specific Si and Fe isotopic compositions of the silicate Earth reflect core partitioning, and if they do, put constraints on important aspects of core formation such as temperature or the presence of Si as a light element in the core. The experimental results will serve as a benchmark for ab initio calculations of Si and Fe isotopic fractionation between relevant metal and silicate phases at high pressure and temperature. The theoretical work will in turn guide and refine the experimental and geochemical modelling efforts, focusing in particular on nuclear resonant measurements, force constant derivations, anharmonic and spin crossover effects. The exchanges and feedbacks between geochemists and experimental and theoretical physicists involved in this project will provide a holistic view of Si and Fe isotopic fractionation during core formation.

首先，地球被分为三个不同性质的同心壳层：金属地核、岩石地幔和流体大气/水圈。虽然可以从地幔和大气/水圈获得样品，但对地核的性质和组成仍然知之甚少。特别是，已知地核比纯铁镍合金密度小，这表明地核中存在另一种轻元素，可能是氧、硅或硫。地核形成的条件（压力和温度）以及地核中轻元素的性质是行星科学中尚未解决的两个主要问题。由于没有可直接用于研究的岩心样本，科学家们依靠远程地震观测或其他间接方法来解决这些问题。在提议的工作中，将结合地球化学（地球化学），矿物物理学（应用于天然材料的固体物理学）和计算技术的方法，对地核形成过程中的温度条件和地核中轻元素的性质进行限制。这将通过研究在地核形成时，不同同位素口味的硅和铁在金属和硅酸盐之间划分的程度来实现。这项工作将包括在实验室合成矿物，并通过将样品限制在两颗钻石之间，将它们压缩到与地球深处相关的压力条件下，在产生高能x射线的同步加速器源上测量这些矿物中铁键的强度，并通过计算机计算来检查物质在高压和高温下的行为。这项工作可以影响许多科学领域，从地球发电机的起源到通过测量它们的质量来表征系外行星。所有pi将积极参与研究生、本科生和博士后的研究项目培训和教育。pi将继续开发SciPhon，这是一款用户友好的免费NRIXS数据缩减软件。该计划将提供给研究NRIXS不同方面的各种社区，包括地球化学、矿物物理、材料科学、凝聚态物理和生物化学。所有pi都将积极参与外展计划，包括UTeach外展计划，该计划为来自奥斯汀和德克萨斯州西南部地区的代表性不足的K-12儿童举办学术夏令营。地球的质量和它的吸积历史使得地核-地幔的分化可能是不可避免的，但对于这种分化是如何以及何时发生的，仍然存在相当大的不确定性。我们对这一重大事件的了解有限，因为我们缺乏对地球深处的采样。科学家们设计了一些间接的方法来解决这一缺陷，他们依靠(1)矿物物理实验来重现地球内部普遍存在的高压-高温条件，(2)理论计算来模拟那些相同的条件，(3)对地幔岩石成分的地球化学测量来寻找地核形成的标志。这些紧密交织的方法已经取得了重大进展，但仍然存在一些一级未解决的问题，例如，在何种压力-温度条件下形成岩心，岩心中轻元素的性质是什么，岩心形成是否分馏出Si和Fe同位素。陆相玄武岩具有非球粒状的Si和Fe同位素组成，这可能反映了这些元素在岩心中的分配，尽管存在其他解释。研究人员建议利用高压核共振非弹性x射线散射（NRIXS）和地幔深部条件下的理论计算，通过地球化学（Dauphas）、理论重新计算（Wentzcovitch）和实验矿物物理（Lin）的合作方法建立Si和Fe同位素分选因子。玄武岩玻璃、下地幔矿物（桥辉石和铁方长石）和铁合金中Si和Fe键的导出力常数将使我们能够建立一个地球深部地球化学模型，以评估硅酸盐地球中特定的Si和Fe同位素组成是否反映了核心分配，如果它们确实反映了核心形成的重要方面，如温度或Si作为轻元素在核心中的存在。实验结果将为在高压和高温下相关金属相和硅酸盐相之间的Si和Fe同位素分馏的从头计算提供基准。理论工作将反过来指导和完善实验和地球化学建模工作，特别关注核共振测量、力常数推导、非谐波和自旋交叉效应。地球化学家与实验和理论物理学家之间的交流和反馈将提供岩心形成过程中Si和Fe同位素分馏的整体视图。