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

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.

This NERC IRF will provide social and cultural benefits to museums, schools and the public by incorporating research outcomes into a refurbished museum gallery, school outreach events and public engagement activities that are expected to reach audiences numbering in the tens of thousands. The main aim of the proposed research - to determine whether the subduction of oceanic lithosphere creates oxygen fugacity (fO2) heterogeneity in the convecting mantle - lies within NERC's remit to support discovery science. Impact will therefore be maximised by realising social and cultural benefits rather than skills, policy or economic benefits. Nevertheless, the new techniques and observations delivered by the proposed research may ultimately provide long-term policy and economic benefits to governments (i.e. volcano observatories), geothermal energy companies and mineral extraction companies by improving estimates of magma storage conditions beneath active volcanoes and refining models of fO2-sensitive ore-forming processes, but these intangible benefits are not considered in detail here.

Manchester Museum will benefit through the incorporation of research outcomes into a refurbished mineral gallery. The museum attracts over 500,000 visitors per year and has received award-winning recognition for its work with the public. It is currently undergoing a £13 million refurbishment, and is expected to attract 700,000 visitors per year when it reopens fully in 2021. The museum's mineral gallery is currently closed as part of the refurbishment, presenting an excellent opportunity to place cutting-edge research at the heart of new displays that will be developed for the reopening. Contributing to the redevelopment of a museum gallery will allow me to share my key outcomes with large numbers of people over many years, ensuring that the proposed research will deliver sustained social and cultural benefits. I will showcase the refurbished mineral galley by organising and delivering family-orientated 'Big Saturday' events on the theme of studying the Earth's interior; 'Big Saturday' events involve hands-on activities and talks from university and museum staff. I will also organise talks and discussions as part of the museum's adult learning programme, 'Museum Lates'.

Local schools will benefit through my involvement with the established Department of Earth and Environmental Sciences' activities for Year 7-11 and Year 12-13 students. I will also explore developing activities for Year 3-6 students, with the aim of inspiring and informing young audiences about the Earth as early in their education as possible.

The wider public will benefit through my involvement in university-wide outreach events throughout the course of the proposed research. The Department of Earth and Environmental Sciences, and the Planetary Science Research Group in particular, make well-established contributions to university-wide outreach events such 'Bluedot' and 'ScienceX' that reach audiences numbering in the tens of thousands. A key strength of these events, which are held at a music festival and shopping centre respectively, is that they engage diverse audiences otherwise unlikely to attend university outreach events. I will also contribute to more traditional outreach events such as the 'Pint of Science' talk series and the 'Manchester Community Festival' that are held annually.

The success of incorporating of research outcomes into a refurbished mineral gallery will be assessed in collaboration with PP Gelsthorpe and Manchester Museum through monitoring museum visitor numbers and ongoing visitor feedback on the refurbished gallery. The University of Manchester has established mechanisms in place to collect feedback on school outreach events and thereby assess their success. They also have established mechanisms in place for documenting the number and nature of interactions during at major outreach events.