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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同时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基本问题。随着板块构造理论在过去50年中的成熟，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在创造成分不均匀性方面发挥了核心作用，我们现在看到，这反映在大洋中脊和海洋岛屿喷发的岩浆的化学变化中。大洋中脊的热液循环使新形成的岩石圈氧化，这意味着俯冲作用将氧气从地球表面输送到其内部深处，从而产生地幔氧含量的变化。然而，目前对喷发的岩浆的观察，提供了进入地球深层化学结构的窗口，提供了相互矛盾的观点，即俯冲是否造成了地幔氧化电位（即氧逸度; fO 2）的异质性，以及氧含量的变化。此外，目前的方法来估计岩浆fO 2往往受到相当大的不确定性，很少可以适用于近原生岩浆，记录最多的信息组成和fO 2的异质性在深度。因此，在本发明中，地幔fO 2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是未知的，尽管事实上，长期行星的可居住性最终取决于喷发的岩浆及其地幔源的fO 2。突出的问题阻碍了我们确定地幔fO 2是否与深部氧循环耦合以及整体评估地球深部在创造一个可居住的行星中的作用包括：-海洋岩石圈的俯冲是否在对流地幔中产生fO 2的不均匀性？-俯冲（即板块构造）的开始是否改变了地幔的fO 2或fO 2结构？在地质年代中，地幔的fO 2或fO 2结构的变化是否在创造地球目前生命所依赖的氧化性大气中发挥了作用？这个NERC IRF的目的是解决这些问题的第一个，确定是否俯冲的海洋岩石圈造成的对流地幔中的fO 2的不均匀性。我将实现这一目标，利用fO 2敏感的形态的Fe（即Fe 3 +/FeT）在单斜辉石提供一个步骤的变化，我们的能力估计fO 2的岩浆和他们的地幔来源，解决四个问题：Q1如何单斜辉石Fe 3 +/FeT在岩浆与地幔源组成？Q2单斜辉石Fe ~（3+）/FeT如何记录地壳岩浆演化过程中的fO ~ 2？Q3在熔融过程中，fO 2和地幔成分如何独立地影响熔体Fe 3 +/FeT？问题4：地幔fO 2与地球构造驱动的深部氧循环耦合到什么程度？我将回答这些问题相结合的高压，高温实验与尖端的X射线吸收近边结构（XANES）光谱，以确定，然后建模的fO 2的Fe 3 +/FeT在岩浆和地幔单斜辉石的依赖性在前所未有的细节。我将校准新的单斜辉石为基础的工具，估计岩浆fO 2，并将其应用到洋中脊，洋岛和火山弧岩浆来自地幔源含有不同数量的俯冲大洋岩石圈。然后，我将涉及到这些岩浆的熔融Fe 3 +/FeT的fO 2的地幔源使用新校准的fO 2敏感的熔融模型。因此，我将提供新的见解fO 2的俯冲带输出，饲料弧和洋岛岩浆作用。此外，开发所需的工具来调查fO 2异质性在目前的地幔将帮助我追求我未来的研究愿景，了解俯冲如何塑造地球的能力，通过地质时间支持生命。