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3.5 Melt Physics and Chemistry

Melt phases are important components throughout the entire Earth and, from past to present, play a central role as means of transport for the chemical and physical differentiation of the Earth into crust, mantle and core. Both laboratory experiments and theoretical calculations are important resources for gaining information about the physical and chemical properties of melts at conditions prevailing in the Earth's interior.

The following contributions address a wide range of questions covering melts relevant to the Earth's crust, mantle and core. Spectroscopy has traditionally been a key tool used to study melts. Raman, infrared, NMR and other spectroscopic studies of silicate glasses quenched from superliquidus temperatures at high pressures have provided important links between melt/glass structure and their properties. Now, updated techniques are being employed to probe melts directly at pressures and temperatures of interest. For example, complex impedance spectroscopy, which has been used successfully in determining the electrical conductivities of mantle minerals, is now used for the investigation of melts and partially molten systems. The presence of a melt, even in minor amounts, can radically alter the bulk electrical properties of a material. Studies of melt viscosity and noble gas solubility at mantle pressures provide constraints on magma transport and rheology, which are important for understanding the development of a magma ocean in the early Earth and the formation of the atmosphere through degassing. Laboratory studies related to properties of the core, with conditions that are extremely challenging to reach experimentally, have placed a greater emphasis on accurately determining the pressure dependence of physical properties of metal melts at more accessible conditions in order to make better estimates of such critical issues as outer core viscosity and composition.

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