The Distribution of Oxygen in Earth's Mantle

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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.