The Distribution of Oxygen in Earth's Mantle

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.

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.