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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表面玄武岩的喷发是地球内外发生化学物质转移的主要机制之一。这种质量转移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”），这会影响地球表面岩石、流体和气体的组成和性质，以及地幔本身的化学和物理性质。例如，富氧大气层的上升与地幔和地表储层之间的氧化和还原化学物质的通量有关。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。地幔岩浆形成过程中控制氧化还原交换的主要矿物是辉石。然而，在地幔部分熔融过程中，辉石中Fe 3+和Fe 2+的相对稳定性知之甚少。该项目旨在通过高温高压实验程序和开发使用X射线光谱学进行辉石微束分析的新方法，确定辉石、岩浆和尖晶石（地幔中另一个重要的铁储层）之间的Fe 3+和Fe 2+交换。这项工作的更广泛的影响，包括研究生培训和本科生的研究经验，并制定和分发的标准和方法的辉石Fe 3+微量分析到其他研究小组。此外，研究成果将通过暑期学校场地纳入地球和行星科学家的跨学科教育，以培训高级研究生，博士后和早期职业科学家。由于缺乏精确校准的微量分析方法，辉石中Fe 3+的测定受到阻碍。在这个项目中，我们将进一步开发基于同步加速器的辉石的X射线吸收光谱，通过表征一套标准品，通过穆斯堡尔光谱分析，并通过应用一种方法，该方法通过使用电子背散射检测定向晶体来解释晶体各向异性。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。这些实验的分析将提供限制上地幔和部分熔融过程中的Fe 3+的分配。这些结果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的Fe 3+的量，因此导致碳在还原相和Fe-Ni合金沉淀物中出现的深度的修正估计。他们还将确定来自不同构造域的玄武岩中Fe 3+浓度之间的关系、玄武岩形成的条件及其来源的氧化状态，从而有助于解决地幔中长期的氧化还原循环和质量平衡问题，该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查进行评估，被认为值得支持的搜索.