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3.7 Melt physics and chemistry

An adequate description of the physics of magmatic processes requires a generalizable set of data on the physical properties of all participating phases. The silicate melt is the essential phase defining the domain of igneous processes. As such the physical properties of silicate melts command a role of central importance in the development of reliable models for igneous evolution and its consequences for the Earth. In recent years interest and activities have grown in several areas of silicate melt physics, including the development of a PVT equation of state for multicomponent liquids, models of the temperature- and composition-dependence of melt viscosity and systematics of melt compressibilities.

Such studies constrain the petrogenetic paths followed by magmatic and volcanic systems in the Earth and other terrestrial planets for a given set of intensive and kinetic parameters.

Nowhere are the effects of subtle variations in the physical and chemical properties of melts on phenomenology of the active Earth more apparent than in the case of explosive silicic volcanism. Here the combined influences, interactions and feedback mechanisms between melt chemistry (e.g., SiO2 content, H2O content), melt properties (e.g., viscosity, surface tension, density) and petrogenetic processes (e.g., conduit flow, degassing kinetics, fragmentation and explosivity of eruptions) lead to radical variations in the eruptive style of intermediate to silicic volcanism which in turn drive mass transfer between the magmatic component of the Earth´s lithosphere and the atmosphere/hydrosphere system. The remarkably non-linear nature of the effect of water on the properties of silicic melts is a central theme in modern volcanology. Understanding the structural and dynamic origins of these property variations is of paramount importance.

Increasing awareness in recent years of the extremely broad range of pressures and temperatures over which magmatism occurs within the Earth and the other terrestrial planets has fuelled interest in the physical and chemical properties of melts under these conditions. At the highest pressures and at the lowest temperatures corresponding to melting and melt fractionation processes in nature the properties of silicate melts are not yet adequately constrained. Thus studies of the pressure and temperature-dependence of the properties of silicate melts are in progress in order to provide generalizable relationships capable of constraining the pressure- and temperature-dependence of melt properties such as density and viscosity. These studies require a sufficient compositional database to permit reliable interpolation to the melt compositions anticipated in nature. The extreme limits of pressure and temperature involved in the Earth´s magmatism can generate exotic melt chemistries, aided in part by the role of disequilibrium during melting and kinetic hindrances to equilibrium fractionation. A broad and systematic approach to the problem is therefore required.

The thermodynamic description of silicate melts remains one of the last outstanding unknown variables in the modelling of melt-crystal equilibria. The activities of melt components cannot yet be simply predicted. In fact, the general form of an ultimately successful melt solution model is not yet agreed upon. Using solid-liquid equilibrium partitioning studies in systems where activity-composition relations for the solid solutions in the solid component are well characterised one may generate such activity data from partitioning studies. In this manner activity-composition relations are being built up for simple and multicomponent melts.

Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Deutschland
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