The Distribution of Oxygen in Earth's Mantle

Lead Research Organisation: Durham University
Department Name: Earth Sciences

Abstract

The exchange of oxygen between the mantle and surface environment is a key component of Earth's geochemical cycle. Oxygen plays a role in generation of magma within the earth and the transfer of volatile elements like sulfur, carbon and hydrogen from the solid earth into molten rock. When such magma rises towards the surface, volcanic gases are released, driving or regulating the composition of Earth's atmosphere. It is therefore increasingly accepted that variation in the oxygen content of Earth's mantle is closely linked to Earth's habitability as a planet.

Basaltic magma is generated by melting the mantle. Therefore, the composition of samples of basaltic volcanoes carries information about the composition of the underlying mantle. However, many processes modify the magma's composition from the point of generation at depth to eruption at the surface. The generation of the melt itself, its crystallisation in the shallow crust and the loss of volcanic gases near the surface all change the composition of magma. It is therefore necessary to account for these processes in order to understand the chemical characteristics of the mantle. Previous attempts to study variation in the oxygen content of the mantle have made simplifying assumptions about these processes. However, progress in theoretical understanding of elemental behaviour indicates that the correcting assumptions need to be revisited. One startling feature of the previous studies is that they come to very different conclusions about the distribution of oxygen in the mantle. The current uncertainty in the oxygen content of the upper mantle corresponds to a number of oxygen atoms that is about 100 times that present in the atmosphere!

We think that part of this discrepancy is caused by the sets of assumptions that previous investigators have made. A crucial component of our project is therefore to use new theoretical and observational constraints to understand how the processes in magmatic systems modify the chemistry of basalt. We have carefully chosen our target geologic setting: Iceland has plentiful basalts that are well studied in terms of traditional chemical compositions and therefore provide us with the extensive background information we need to underpin our models. Once we have improved models by adding our new observations, we can better focus our map of the variation in the oxidation state of the Earth's mantle.

In detail, our research will involve a great deal of painstaking geochemical work. We aim to use the isotopic composition of the element vanadium, because theoretical work and preliminary experimental studies indicate that the behaviour of vanadium and its isotopes is strongly controlled by mantle oxidation state. By combining new constraints from vanadium isotopes with other geochemical measurements that are thought to be sensitive to mantle oxygen, we can construct a model of oxidation state across the Iceland. The combination of several independent chemical constraints allows us to determine just how much variation in oxygen there is beneath this classic locality. Furthermore, it equips the community with a precise tool to extract global variations in mantle oxidation.

Planned Impact

Industrial Beneficiaries:
-FEI (high performance microscopy company) - Direct benefit;
-Mineral Resource and Energy companies - Indirect benefit.

FEI sells many QEMSCAN imaging systems to industry every year. The importance of quantifying the mineral compositions in ore rocks is clear for mining companies, and hydrocarbon companies use QEMSCAN for understanding reservoir quality. QEMSCAN converts EDS data into mineral-composition maps using a SIP file. We will develop a SIP file for mafic/ultramafic rocks and will share this with FEI. They may then use this file in combination with other companies to improve characteristion of ore-bearing rocks, with crucial global economic influence. We will also generate beautiful high resolution images that FEI may use in their marketing of the QEMSCAN system. The project PDRA and the summer students on the project will develop experience with QEMSCAN - a skill that will be very attractive to mining and hydrocarbon companies that use the software.

Educational Beneficiaries:
-Public/undergraduates/postgraduates via Imperial Rock Library -Direct;
-Industries requiring petrographic ability - Indirect.
-UK education via promotion of STEM subjects - Direct

The Imperial College London Rock Library is a database of rocks and minerals specifically designed as a university level training, evaluation and reference resource. Through easy access to thousands of examples of minerals, textures and rock types, straightforward online identification tools, training activities and evaluation tests the Rock Library provides a unique set of tools to ensure Earth Scientists obtain and maintain a high level of practical ability without requiring direct access to microscopes and hand specimens. The Rock Library and currently attracts 400,000 users per year from almost every country, most of which are undergraduates, postgraduates and industry geologists. However, there is a lack of basaltic examples in general and Icelandic examples specifically. Our work will address this by making high quality QEMSCAN images available to the Rock Library, thus expanding its technologic repertoire.

Encouraging the next generation of scientists is a fundamental responsibility of academia. We approach this by using 'student reporters' who will document the course of the work by participation in field work and subsequently following the samples they have witnessed being collected as they are converted into high quality, meaningful chemical data. Student reporters will inform both their peers and the general public through outreach including, but not limited to, the Imperial Festival, blogs, vlogs, and Imperial's Earth Class. Earth Class is specifically aimed at year 9 and 10 students who will be considering further education. We hope to generate excitement and enthusiams for STEM subjects in general, and our work in particular by portraying the journey of the work through the eyes of the young student reporters.

Publications

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Related Projects

Project Reference Relationship Related To Start End Award Value
NE/N009568/1 01/08/2016 31/03/2018 £66,251
NE/N009568/2 Transfer NE/N009568/1 01/04/2018 31/12/2019 £45,989
 
Description In this research, we try to understand the amount of available oxygen and how it varies in the Earth. The amount of available oxygen in Earth Sciences is often referred to in terms of its partial pressure, or fugacity. Oxygen fugacity varies by over 9 orders of magnitude in the Earth and there are many independent approaches determining its value. However, these approaches are often in disagreement. Therefore, this research takes 3 independent approaches (V/Sc ratios, Fe3+/Fe sum ratios and V isotopic compositions) to determining fO2 on the same samples to see which approach works best and to try and understand why the three approaches do not always give the same answer.
Exploitation Route Academic: Understanding the drivers of isotopic fractionation has wide reaching impacts, for example, finding new ways to label chemicals, material sciences in terms of better understanding crystal structures. Stable isotopic fractionation of transition metals is increasingly used to trace contaminants through the environment.

Non-academic: The undergraduate helpers we had on Icelandic fieldwork have continued in academia and are now both undertaking PhD degrees.
Sectors Chemicals,Education,Environment

 
Description The experience gained during fieldwork by our undergraduate helpers (not directly funded by the grant, but part of the impact plan) has encouraged them continue on a path to PhD degrees
Sector Education
Impact Types Societal

 
Description BGI Research Grant - Vanadium stable isotope fractionation in Earth's Interior
Amount € 2,000 (EUR)
Organisation German Research Foundation 
Sector Charity/Non Profit
Country Germany
Start 02/2018 
End 02/2018
 
Description IAPETUS DTP PhD studentship to M. Stow
Amount £5,338,027 (GBP)
Funding ID NE/L002590/1 
Organisation Durham University 
Sector Academic/University
Country United Kingdom
Start 10/2018 
End 03/2021
 
Description Cheltenham Science Festival 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Schools
Results and Impact PDRA Davide Novella participated in the GeoBus outreach research program at the 2017 Cheltenham Science Festival in collaboration with University College London.
Year(s) Of Engagement Activity 2017
 
Description Departmental Seminar at University of Portsmouth, UK 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Professional Practitioners
Results and Impact I gave an invited departmental seminar specifically on the outcomes of my NERC fellowship - stable vanadium isotope fractionation at high temperatures
Year(s) Of Engagement Activity 2014
 
Description Departmental Seminar, Hannover, Germany 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Postgraduate students
Results and Impact Invited departmental seminar at the Institute fur Mineralogie and Petrologie, Hannover, Germany. A 45 minute lecture with lively 20 minute discussion afterwards as part of a two-day visit to initiate and strengthen international collaboration.
Year(s) Of Engagement Activity 2016
 
Description Undergraduate field work student reporters 
Form Of Engagement Activity Engagement focused website, blog or social media channel
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Undergraduate students
Results and Impact As per our impact plan, we recruited two undergraduate students to join us for field work associated with this grant. The students, Xenia and Kevin, took video of our fieldwork, posted regular blog and twitter entries during the trip: (blog): https://lavatolab.wordpress.com (twitter) https://twitter.com/lavatolab. They will continue to be engaged with the grant progression, telling the story of how a collected lava can eventually tell us about the amount of oxygen in Earth's mantle.
Year(s) Of Engagement Activity 2016