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

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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.