Constraining core-mantle interaction

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Some 3000km beneath the surface of the Earth the temperature jumps 2000K and the surroundings change from solid silicate minerals to liquid iron alloy. This is the core-mantle boundary. It is our planet's most profound discontinuity, with huge physical and chemical changes occuring over a very small distance (<200m). The temperature contrast across the boundary has long been held responsible for heating the mantle above the core such that it becomes less dense and upwells as plumes to create oceanic islands such as Hawaii. However, this long held 'plume paradigm' has been increasingly questioned and some hard evidence for the an ultimately deep origin of the material that forms these oceanic islands is needed. The traditional tool for imagining the Earth's interior, seismology, has difficulty in clearly resolving narrow plumes beneath ocean islands. Geochemistry could provide the crucial evidence if a chemical inprint of residence next to the compositonally highly distinctive core could be found in oceanic island lavas. Recent work has suggested that there are such signatures. Initial work focussed on elevated values of an isotope ratio of the element Os, 186Os/188Os. Elevated 186Os/188Os ratios were found in lavas from Hawaii and can be attractively explained by a contribution from the core in their source. However there are other plausible explanations of this observation. A much less equivocal fingerprint comes from another isotope ratio, this time of the element W, 182W/184W. This ratio changed only in the first 50My of Earth's ~4550My history and so only reflects the most ancient processes that originally established the compositional differences between core and mantle. The W isotope system is also highly sensitive to the addition of core material to the mantle but an initial study indicated no core W isotope signature in the samples from Hawaii which showed the most extreme Os isotope compositions. The work here proposes to resolve this empasse. A plausible alternative needs to be found for the elevated 186Os/188Os and objections raised to the reliability of the W isotope tracer need to be fully addressed. Moreover, the approach needs to be used further than the current database of three samples from a single ocean island. This project will study a large range of locations where geophysical or geochemical evidence has suggested an ultimately deep source for magmatism. In addition a new tracer will be used, namely Mo. Using analyses of the concentration of Mo and its isotopes it is possible to clearly distinguish between several competing hypotheses to account for the contrasting W and Os isotopic data. The outcome of the study will provide a definitive answer as to whether we can chemically sample the Earth's deepst reservoir at the planet's surface. As indicated above, this is not idle speculation but will help us understand how volcanism in the middle of tectonic plates occurs and indeed what processes occur at the most dramatic boundary on Earth.