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

合作项目：CSEDI-通过地球化学和矿物物理学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: 1502594
• 负责人: Jung-Fu Lin
• 金额: $ 22.63万
• 依托单位: University of Texas at Austin
• 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 2020-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1502594&HistoricalAwards=false
• 关键词: Collaborative; project; 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：金属核，岩石地幔和流体大气圈/水圈。虽然可以从地幔和大气圈/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，已知核心的密度低于纯铁镍合金，表明核心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）和地核中轻元素的性质是行星科学中两个尚未解决的主要问题。由于没有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。在拟议的工作中，地球化学（地球化学），矿物物理学（应用于天然材料的固态物理学）和计算技术的方法将结合起来，以确定核心形成期间的温度条件和地球核心中轻元素的性质。这将通过检查在核心形成时硅和铁的不同同位素在金属和硅酸盐之间分配的程度来实现。这项工作将涉及在实验室合成矿物，并通过将样品限制在两个钻石之间，将它们压缩到与地球深部相关的压力条件下，在产生高能X射线的同步加速器源上测量这些矿物中的铁键强度，并通过计算机计算检查高压和高温下物质的行为。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太阳系外行星的质量来描述它们的特征。 所有PI将积极参与培训和教育研究生，本科生和博士后在拟议的研究项目。 PI将继续开发SciPhon，这是一款用户友好的免费软件，用于NRIXS数据简化。该计划将提供给研究NRIXS不同方面的各个社区，包括地球化学、矿物物理学、材料科学、凝聚态物理学和生物化学。 所有的PI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是这样的，核幔分化可能是不可避免的，但相当大的不确定性仍然是如何以及何时发生的。我们对这一重大事件的了解有限，这是因为我们缺乏对地球内部深处的取样。科学家们设计了间接的方法来解决这个缺点，依靠（1）矿物物理实验来重现地球内部普遍存在的高压-温度条件，（2）理论计算来模拟这些相同的条件，（3）地幔岩石成分的地球化学测量来寻找核心形成的迹象。这些强烈交织的方法导致了重大进展，但一阶未回答的问题仍然存在，例如在什么压力-温度条件下核心形成，核心中轻元素的性质是什么，以及核心形成的裂变Si和Fe同位素。陆地玄武岩具有非玄武岩的Si和Fe同位素组成，这可能反映了这些元素进入核心的分区，尽管存在其他解释。研究人员建议使用高压核共振非弹性X射线散射（NRIXS）和理论计算，通过地球化学（Daufas），理论从头计算（Wentzcovitch）和实验矿物物理学（Lin）的合作方法，在深地幔条件下建立Si和Fe同位素分馏因子。下地幔矿物玄武玻璃中Si和Fe键的力常数（硼镁石和铁方镁石）和铁合金将使我们能够建立一个深地球地球化学模型，以评估硅酸盐地球的特定Si和Fe同位素组成是否反映核心分配，如果是这样，限制了核心形成的重要方面，例如温度或核心中作为轻元素的Si的存在。实验结果将作为一个基准的从头计算的硅和铁同位素分馏相关的金属和硅酸盐相在高压和高温。理论工作将反过来指导和完善实验和地球化学建模工作，特别侧重于核共振测量，力常数推导，非谐和自旋交叉效应。参与该项目的地球化学家和实验及理论物理学家之间的交流和反馈将提供对岩芯形成过程中Si和Fe同位素分馏的整体看法。