Mantle redox and partial melting: Pyroxene/basalt and pyroxene/spinel partitioning of Fe3+

地幔氧化还原和部分熔融：Fe3 的辉石/玄武岩和辉石/尖晶石分配

基本信息

批准号：
2016215
负责人：
Marc Hirschmann
金额：
$ 46.7万
依托单位：
University of Minnesota-Twin Cities
依托单位国家：
美国
项目类别：
Standard Grant
财政年份：
2020
资助国家：
美国
起止时间：
2020-07-15 至 2024-06-30
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=2016215&HistoricalAwards=false
关键词：
Mantle redox partial melting Pyroxene

项目摘要

Formation of magma in Earth’s mantle and eruption at the surface as basalt is one of the chief mechanisms by which chemical mass transfer occurs between the interior and exterior of the planet. A key feature of this mass transfer is exchange of oxidized and reduced chemical species (“redox exchange”), which influences the composition and properties of rocks, fluids, and gases at Earth’s surface, as well as the chemical and physical properties of the mantle itself. For example, the rise of an atmosphere rich in oxygen is linked to the fluxes of oxidized and reduced chemical species between Earth’s mantle and surface reservoirs. The principal element responsible for redox exchange during these processes is iron, which can take on both oxidized (Fe3+) and reduced (Fe2+) forms. The chief mineral controlling redox exchange during magma formation in the mantle is pyroxene. However, the relative stability of Fe3+ and Fe2+ in pyroxene during partial melting of the mantle is poorly understood. This project aims to determine exchange of Fe3+ and Fe2+ between pyroxene, magma, and spinel, another important iron reservoir in the mantle, through a program of high temperature high pressure experiments and by developing new methods for microbeam analysis of pyroxene using X-ray spectroscopy. The broader impacts of this work include graduate student training and research experiences for undergraduates, and development and distribution of standards and methods for pyroxene Fe3+ microanalysis to other research groups. Also, research results will be incorporated into interdisciplinary education of earth and planetary scientists through summer school venues for training of advanced graduate students, post-docs, and early-career scientists.Determination of Fe3+ in pyroxene has been hampered by the absence of an accurately calibrated microanalytical method. In this project, we will further develop synchrotron-based x-ray absorption spectroscopy of pyroxene by characterizing a suite of standards, analyzed by Mössbauer spectroscopy, and by applying a method that accounts for crystallographic anisotropy by orienting crystals using electron backscatter detection. We will conduct high temperature high pressure experiments in which pyroxenes are in equilibrium with basaltic melt or with spinel. Analysis of these experiments will provide constraints on the partitioning of Fe3+ in the upper mantle and during partial melting. These will be applied to several key problems in petrology and high temperature geochemistry. They will constrain the amount of Fe3+ in mantle source regions of the mantle, and therefore lead to revised estimates of the depths at which carbon occurs in reduced phases and Fe-Ni alloy precipitates. They will also determine the relationship between Fe3+ concentrations in basalts from different tectonic domains, the conditions of basalt formation, and the oxidation state of their source, thereby helping to resolve long-term redox cycling and mass balance in the mantle-surface system.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.

地幔中岩浆的形成和地表玄武岩的喷发是地球内部和外部之间发生化学物质传递的主要机制之一，这种物质传递的一个关键特征是氧化和还原化学物质的交换。 “氧化还原交换”），它影响地球表面岩石、流体和气体的成分和性质，以及地幔本身的化学和物理性质，例如，富含氧气的大气层的上升与此有关。地球地幔和地表储层之间氧化和还原化学物质的通量，在这些过程中负责氧化还原交换的主要元素是铁，它可以呈现氧化 (Fe3+) 和还原 (Fe2+) 形式，是控制氧化还原交换的主要矿物。然而，对于地幔部分熔融过程中辉石中 Fe3+ 和 Fe2+ 的相对稳定性知之甚少。通过高温高压实验计划和开发使用 X 射线光谱对辉石进行微束分析的新方法，研究了辉石、岩浆和尖晶石（地幔中另一个重要的铁储层）之间的 Fe3+ 和 Fe2+。包括研究生培训和本科生研究经验，以及辉石 Fe3+ 微量分析标准和方法的开发和分发给其他研究小组。此外，研究成果将纳入地球和行星科学家的跨学科教育。通过暑期学校场地培训高级研究生、博士后和早期职业科学家。辉石中 Fe3+ 的测定因缺乏精确校准的微分析方法而受到阻碍。在该项目中，我们将进一步开发基于同步加速器的方法。通过表征一套标准品的辉石 X 射线吸收光谱，通过穆斯堡尔光谱进行分析，并应用一种解释晶体各向异性的方法我们将进行高温高压实验，其中辉石与玄武岩熔体或尖晶石处于平衡状态，这些实验的分析将为 Fe3+ 在上地幔和部分熔融过程中的分配提供限制。将应用于岩石学和高温地球化学中的几个关键问题，它们将限制地幔源区中 Fe3+ 的含量，从而导致对碳出现深度的修正估计。他们还将确定来自不同构造域的玄武岩中 Fe3+ 浓度、玄武岩形成条件及其来源的氧化态之间的关系，从而有助于解决长期氧化还原循环和氧化态的问题。该奖项反映了 NSF 的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。