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Oxygen fugacity in a heterogeneous mantle: Earth's deep oxygen cycle

异质地幔中的氧逸度：地球深部氧循环

基本信息

批准号：
NE/T011106/1
负责人：
David Neave
金额：
$ 85.66万
依托单位：
University of Manchester
依托单位国家：
英国
项目类别：
Fellowship
财政年份：
2021
资助国家：
英国
起止时间：
2021 至无数据
项目状态：
未结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FT011106%2F1
关键词：
Oxygen fugacity heterogeneous mantle Earth

项目摘要

Is it a coincidence that Earth is the only planet on which both life and plate tectonics are known to exist? This is the fundamental question that motivates my future research vision. As plate tectonic theory has matured over the past 50 years, we have come to increasingly appreciate our planet's dynamism and complexity. Tectonic subduction of oceanic lithosphere over billions of years has played a central role in creating the compositional heterogeneity we now see reflected in the chemical variability of magmas erupted at mid-ocean ridges and ocean islands. Hydrothermal circulation at mid-ocean ridges oxidises newly formed lithosphere, meaning that subduction transports oxygen from Earth's surface into its deep interior, generating variability in mantle oxygen contents. However, current observations from erupted magmas, which offer windows into Earth's deep chemical structure, provide conflicting views on whether subduction has created heterogeneity in the oxidising potential (i.e. oxygen fugacity; fO2) of the mantle alongside variations in oxygen content. Moreover, current approaches for estimating magma fO2 are often subject to considerable uncertainties and can rarely be applied to the near-primary magmas that record the most information about compositional and fO2 heterogeneity at depth. Thus, the extent to which variations in mantle fO2 are coupled to the tectonically driven deep oxygen cycle remains largely unknown despite the fact that the fO2 of the volcanic gases that mediate long-term planetary habitability is ultimately determined by the fO2 of erupted magmas and their mantle sources.Outstanding problems impeding our ability to determine whether mantle fO2 is coupled to the deep oxygen cycle and holistically evaluate the deep Earth's role in creating a habitable planet include:- Does the subduction of oceanic lithosphere create fO2 heterogeneity in the convecting mantle?- Did the onset of subduction (i.e. plate tectonics) change the fO2 or fO2 structure of the mantle?- Did changes in the fO2 or fO2 structure of the mantle over geological time play a role in creating the oxidising atmosphere upon which much of Earth's current life depends?This NERC IRF aims to resolve the first of these problems by determining whether the subduction of oceanic lithosphere creates fO2 heterogeneity in the convecting mantle. I will achieve this by exploiting the fO2-sensitive speciation of Fe (i.e. Fe3+/FeT) in clinopyroxene to provide a step change in our ability estimate the fO2 of magmas and their mantle sources by addressing four questions:Q1 How does clinopyroxene Fe3+/FeT in magmas relate to mantle source composition?Q2 How does clinopyroxene Fe3+/FeT record fO2 during magmatic evolution in the crust?Q3 How do fO2 and mantle composition independently affect melt Fe3+/FeT during melting?Q4 To what extent is mantle fO2 coupled to Earth's tectonically driven deep oxygen cycle?I will answer these questions by combining high-pressure, high-temperature experiments with cutting-edge X-ray absorption near edge structure (XANES) spectroscopy to determine and then model the fO2 dependence of Fe3+/FeT in magmatic and mantle clinopyroxenes in unprecedented detail. I will calibrate new clinopyroxene-based tools for estimating magma fO2 and apply them to mid-ocean ridge, ocean island and volcanic arc magmas derived from mantle sources containing different amounts of subducted oceanic lithosphere. I will then relate the melt Fe3+/FeT of these magmas to the fO2 of their mantle sources using newly calibrated fO2-sensitive melting models. I will thus provide new insights into the fO2 of the subduction zone outputs that feed both arc and ocean island magmatism. Furthermore, developing the tools required to investigate fO2 heterogeneity in the present mantle will help me to pursue my future research vision of understanding how subduction has shaped Earth's ability to support life through geological time.

地球是唯一一个已知存在生命和板块构造的行星，这是巧合吗？这是激发我未来研究愿景的根本问题。在过去的50年里，随着板块构造理论的成熟，我们越来越认识到地球的活力和复杂性。数十亿年来，海洋岩石圈的构造俯冲作用在形成成分不均匀性方面发挥了核心作用，我们现在看到，中洋脊和海洋岛屿喷发的岩浆的化学变化反映了这种不均匀性。洋中脊的热液循环氧化了新形成的岩石圈，这意味着俯冲作用将氧气从地球表面输送到其内部深处，从而产生了地幔氧含量的变化。然而，目前对喷发岩浆的观测提供了了解地球深层化学结构的窗口，对于俯冲是否造成了地幔氧化电位（即氧逸度；fO2）的不均匀性以及氧含量的变化，提供了相互矛盾的观点。此外，目前估算岩浆fO2的方法往往存在相当大的不确定性，很少能应用于近原生岩浆，而近原生岩浆在深度上记录了最多的成分和fO2非均质性信息。因此，地幔fO2的变化在多大程度上与构造驱动的深部氧循环相耦合仍然很大程度上未知，尽管调节行星长期可居住性的火山气体的fO2最终由喷发岩浆及其地幔源的fO2决定。阻碍我们确定地幔fO2是否与深部氧循环耦合以及整体评估地球深部在创造宜居行星中的作用的突出问题包括：—海洋岩石圈的俯冲是否在对流地幔中造成了fO2的非均匀性？俯冲的开始（即板块构造）是否改变了地幔的fO2或fO2结构？-在地质时期，地幔的fO2或fO2结构的变化是否在创造氧化大气中发挥了作用，而地球当前的大部分生命都依赖于氧化大气？这个NERC IRF旨在通过确定海洋岩石圈的俯冲是否在对流地幔中产生fO2非均质性来解决第一个问题。我将通过利用斜斜辉石中对fO2敏感的Fe形态（即Fe3+/FeT）来实现这一目标，通过解决以下四个问题，为我们估计岩浆及其地幔源的fO2提供一个阶跃变化的能力：Q1斜斜辉石中岩浆中的Fe3+/FeT如何与地幔源组成相关？地壳岩浆演化过程中斜辉石Fe3+/FeT如何记录fO2 ？在熔融过程中，fO2和地幔成分如何独立影响熔体Fe3+/FeT ？Q4地幔fO2在多大程度上与地球构造驱动的深部氧循环耦合？我将通过结合高压、高温实验和尖端的x射线吸收近边缘结构（XANES）光谱来回答这些问题，以前所未有的细节确定Fe3+/FeT在岩浆和地幔斜辉石中对fO2的依赖，然后建立模型。我将校正基于斜辉石质的估算岩浆fO2的新工具，并将其应用于来自地幔源的洋中脊、洋岛和火山弧岩浆，这些地幔源含有不同数量的俯冲海洋岩石圈。然后，我将使用新校准的fO2敏感熔融模型，将这些岩浆的熔体Fe3+/FeT与地幔源的fO2联系起来。因此，我将为俯冲带输出的fO2提供新的见解，这些输出为弧和洋岛岩浆活动提供动力。此外，开发研究当前地幔中fO2异质性所需的工具将有助于我追求未来的研究愿景，即了解俯冲作用如何塑造了地球在地质时期支持生命的能力。