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3.8 Structure and dynamics of silicate melts

A vast assortment of ongoing experimental studies are focused on characterizing the structural and dynamical properties of silicate melts. Designed primarily to examine simpler analogue melts rather than multicomponent natural systems, these studies have improved our understanding of melt structure and dynamics to a level at which we can begin developing atomistic models to describe the complex behaviour of magmas. The summaries given below represent a broad sampling of efforts to accomplish this task.

With a dynamics approach, timescales associated with differentiation processes such as magma mixing, assimilation, and crystal fractionation can be constrained by knowledge of atom mobilities within melts. Viscosity studies provide information related to deep Earth processes such as the separation of partial melt from the source region and magma transport in the mantle and lower crust. Melt viscosity is of particular importance for understanding the evolution and differentiation of a possible magma ocean during the Earth's early history. Viscosity also plays a critical role at the surface in terms of understanding volcanism, from explosive Plinian eruptions to effusive lava flows.

Characterizing silicate melt structure serves as a link to understand rheological and thermodynamic properties. Technological advancements, most notably in areas of in-situ high temperature and high pressure spectroscopies, have expanded our capability of acquiring detailed atomic level structural data over a wide range of experimental conditions. Structural investigations are aimed at understanding viscosity-density relationships, solubility and speciation of various melt components, and can often offer insights into melt-melt and mineral-melt relationships, such as element partitioning and immiscibility of liquids.

An added complexity to melt structure and dynamics is the influence of volatile components like water, carbon dioxide, and fluorine. Water, the most abundant volatile in natural systems, can profoundly effect melt properties such as viscosity, phase relations, liquidus/solidus temperatures, and redox equilibria (e.g. Fe2+/Fe3+). Examination of hydrous melts and glasses can even lead to a more complete understanding of "dry" systems, and the mechanisms which govern the incorporation of water into silicate melts.

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