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3.2 Geochemistry at extreme pressures and temperatures

One of most important geochemical events in the 4.5 billion year history of the Earth was the formation of the iron-rich metallic core. This process involved separation of Fe-rich liquid metal from the silicates and oxides which now constitute the Earth's mantle. It is probable that the metal was in chemical equilibrium with the silicate phases during segregation. Thus, the present geochemistry of the mantle, which can be observed through studies of mantle xenoliths (fragments brought rapidly to the surface by volcanic eruptions), is a reflection of core formation. Metal segregation may have taken place during a magma ocean stage, when a significant part of the Earth was molten, following a postulated collision with a large (Mars-sized) body. Alternatively liquid metal may have segregated from mainly crystalline silicates and oxides. Therefore studies of element partitioning between liquid metal, liquid silicate and crystalline phases of the Earth's mantle are central to understanding core formation. Early element partitioning studies performed at atmospheric pressure suggested that siderophile elements (elements which partition strongly into the metal phase) should have been largely removed from the mantle by the segregating metal during core formation. In contrast to this prediction, the concentration of such elements in the mantle is too high. This discrepancy (the "siderophile element anomaly") has been explained by models of heterogeneous accretion in which the present mantle composition is partly determined by a thin layer (late veneer) accreted onto the Earth after core formation. An alternative possibility is that partitioning behaviour changes at high pressures and/or very high temperatures and that the geochemistry of the present mantle can indeed be explained by equilibrium during metal extraction. The partitioning of various elements is therefore being studied at pressures up to 25 GPa (equivalent to a depth of 750 km) and temperatures of 2500 K using the multianvil apparatus. The effects of oxygen fugacity and silicate liquid composition on element partitioning are also being investigated systematically at high pressure and also at 1 bar through element solubility studies. The conditions under which metallic and sulfide melts can segregate from crystalline and molten silicates is evaluated through textural studies of partially molten samples.

A knowledge of oxygen fugacity in the mantle is important because this parameter controls transport properties (i.e. rheology, diffusion, electrical conductivity, and reaction kinetics). The oxygen fugacity of the mantle is investigated based on the relative abundances of Fe2+ and Fe3+ in minerals which have been brought to the surface as inclusions in diamonds. This work parallels investigations of Fe2+/ Fe3+ ratios in high-pressure phases which have been synthesised experimentally under known conditions in the laboratory.

Phase equilibrium studies of hydrous phases have been performed at high pressure in order to understand the transport of elements into subduction zones and their subsequent recycling into the mantle, as well as the depth to which H2O can be transported by subducting lithosphere. Results are important for understanding the geochemistry of subduction-related volcanism and the occurrence of intermediate-depth earthquakes which are believed to be induced by fluids released during dehydration reactions.

Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany
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