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

地幔岩浆的形成和表面玄武岩的喷发是地球内部和外部发生化学物质传递的主要机制之一。这种质量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，富氧大气的上升与地球地幔和地表储层之间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，它可以以氧化（Fe3+）和还原（Fe2+）形式存在。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Fe3+和Fe2+的相对稳定性尚不清楚。本项目旨在通过高温高压实验和x射线光谱分析辉石微束的新方法，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Fe3+和Fe2+交换。本工作的广泛影响包括研究生培训和本科生的研究经验，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，研究成果将通过培养高级研究生、博士后和早期职业科学家的暑期学校，纳入地球和行星科学家的跨学科教育。由于缺乏精确校准的微量分析方法，辉石中Fe3+的测定一直受到阻碍。在这个项目中，我们将进一步发展基于同步加速器的辉石x射线吸收光谱，通过表征一套标准，通过Mössbauer光谱分析，并通过使用电子背散射检测取向晶体来解释晶体各向异性的方法。我们将进行辉石与玄武岩熔体或尖晶石平衡的高温高压实验。这些实验的分析将为上地幔和部分熔融过程中Fe3+的分配提供约束。这些方法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域中Fe3+的数量，从而导致对碳以还原相和Fe-Ni合金析出的深度的修正估计。他们还将确定不同构造域玄武岩中Fe3+浓度、玄武岩形成条件及其来源氧化状态之间的关系，从而有助于解决地幔-地表系统的长期氧化还原循环和质量平衡问题。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.