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3.2 Experimental Geochemistry

Geochemical studies of Earth materials provide an understanding of processes occurring from the core to the crust and from the early formation of the Earth to the present day. The formation of the Earth's core early in the history of the planet remains poorly understood. While we know that the core is predominantly an iron/nickel alloy with some light element component, the identity of the light element(s) is still unresolved. As incorporation of light elements, such as Si and O, into the liquid metal presumably occurred during growth of the core, these elements must be soluble at relatively low pressure, as well as at the higher pressures and temperatures of the present core. Indications of whether core formation occurred before the Earth attained its current mass are provided by experimental measurements of the partitioning of highly siderophile elements between molten silicate and metal. Significant deviations of partition coefficients for the Earth from experimental values may indicate that core formation occurred prior to the accretion of a later chondritic veneer.

Point defects within minerals, including trace impurities and water, control many of the physical, chemical and mechanical processes within the interior of the Earth. Thus, in order to use laboratory measurements to constrain the properties of the deep mantle, we must quantify the effects of impurity atoms, such as aluminium and ferric iron, on the defect structure of silicate perovskite. Similarly the defect structure of olivine significantly affects its behavior. Small amounts of dissolved nickel narrow the stability field of olivine dramatically under reducing conditions. The interior of the Earth is also an important reservoir for water. Within the upper mantle, a significant quantity of this dissolved water is contained within hydrous minerals in the interiors of subducting slabs. It is important to determine the stability of these hydrous minerals as water released from their decomposition will have major effects on the properties of mantle rocks. Magmas generated by partial melting of upper mantle rocks containing hydrous minerals may be an important source of ultramafic melts. By contrast, melts fluxing through the mantle can react with minerals such as orthopyroxene to generate disequilibrium melt compositions that are quite silica-rich. Melts can also entrain pieces of the upper mantle as xenoliths whose compositions provide information about mantle metasomatism.

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