Magmatism and the evolution of the Earth's interior

Abstract

The flow of liquids relative to solids or other liquids is the only efficient mechanism for obtaining physical separation of chemical components of an initially homogenous material at high pressure. Hence the history of differentiation of the Earth's interior is essentially the history of magmatic phenomena: core formation involving molten metal, formation and freezing of a silicate magma ocean, and ongoing partial melting of silicate solids. In early history much of this activity took place at very high pressure, in the lower mantle (plausibly, silicate magmatic activity continues today at the core-mantle boundary). In order to define possible evolutionary paths of the young Earth and to learn what evidence might remain today of such early processes we must build well-constrained models of igneous processes at appropriate pressures, temperatures, and compositions. This remains difficult because experimental study of lower mantle igneous petrology lies mostly beyond the pressure capability of multi-anvil devices and beyond the size and homogeneity capability of diamond anvil cells. We must rely on indirect tools: construction of phase diagrams and thermodynamic models from thermochemical and equation of state data contributed by mineral physics and the emerging field of high-pressure melt physics. There are roles in this enterprise for quantum and molecular dynamics computation, shock wave experiments, and a variety of in situ applications of synchrotron radiation. Once the major element phase equilibria are well-defined, then it is appropriate to turn to trace elements and the determination of partition coefficients at appropriate pressure, temperature, and major element compositions.

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