Silicate melts: Relaxation, rheology, and the glass transition

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  • SWSharon Webb
    Sharon Webb
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Abstract
Movement of magma within the Earth depends upon the density, compressibility, thermal expansion, viscosity, diffusivity, heat capacity, and thermal conductivity of silicate melts as a function of temperature, pressure, and composition. These physical properties are controlled by the atomic coordination and bond strengths of the melt. To contribute more to our understanding of geological processes than numerous measurements of physical properties of individual magmas of local interest, the relationship between melt structure and physical properties needs to be understood. In geological settings where the viscosity of the molten component is high (108‐1014 Pa s), the melt structure requires a long time (seconds to weeks) to equilibrate in response to changes in pressure, temperature, and composition. The effects of this structural equilibration upon the viscosity, density, and compressibility of magmas and the time required for equilibration need to be included in discussions of magma ascent and emplacement and of crystal nucleation, growth, and segregation within magmas. The structural relaxation discussed here is the slowest relaxation process occurring in a silicate melt at a specific temperature and therefore represents the glass transition, which is a kinetic transition whose temperature is not a constant specific to a melt composition but depends upon the timescale on which measurements are performed. On the basis of the agreement between nuclear magnetic resonance measurements of Si‐O bond exchange timescales and measured structural relaxation timescales, the structural equilibration observed in silicate melts appears to be due to the lifetime of the Si‐O bonds in the melt. The following is a discussion of a range of different studies to determine the density, viscosity, thermal expansion, and compressibility of silicate melts, based on a central theme of the structural relaxation phenomenon and its use to calculate the physical properties of a melt, for example, using calorimetry data to calculate viscosity, volume, and thermal expansion in silicate melts at temperatures 50 K above the glass transition. This results in the determination of melt properties at conditions of direct geological interest, for example, the densities of granitic melts at 1050 K and viscosities of 109–1012 Pa s; observations of anomalously low, non‐Newtonian viscosities and compressibilities in melts at conditions of high strain rate; the viscosities of volatile‐bearing melts; and the viscosity of the melt phase in natural crystal‐ and bubble‐bearing obsidians.

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