Thermal Diffusivity of Rhyolitic Glasses and Melts: Effects of Temperature, Crystals and Dissolved Water

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Thermal diffusivity (D) was measured using laser-flash analysis on pristine and remelted obsidian samples from Mono Craters, California. These high-silica rhyolites contain between 0.013 and 1.10 wt% H2O and 0 to 2 vol% crystallites. At room temperature, D glass varies from 0.63 to 0.68 mm2 s−1, with more crystalline samples having higher D. As T increases, D glass decreases, approaching a constant value of ~0.55 mm2 s−1 near 700 K. The glass data are fit with a simple model as an exponential function of temperature and a linear function of crystallinity. Dissolved water contents up to 1.1 wt% have no statistically significant effect on the thermal diffusivity of the glass. Upon crossing the glass transition, D decreases rapidly near ~1,000 K for the hydrous melts and ~1,200 K for anhydrous melts. Rhyolitic melts have a D melt of ~0.51 mm2 s−1. Thermal conductivity (k = D·ρ·C P) of rhyolitic glass and melt increases slightly with T because heat capacity (C P) increases with T more strongly than density (ρ) and D decrease. The thermal conductivity of rhyolitic melts is ~1.5 W m−1 K−1, and should vary little over the likely range of magmatic temperatures and water contents. These values of D and k are similar to those of major crustal rock types and granitic protoliths at magmatic temperatures, suggesting that changes in thermal properties accompanying partial melting of the crust should be relatively minor. Numerical models of shallow rhyolite intrusions indicate that the key difference in thermal history between bodies that quench to obsidian, and those that crystallize, results from the release of latent heat of crystallization. Latent heat release enables bodies that crystallize to remain at high temperatures for much longer times and cool more slowly than glassy bodies. The time to solidification is similar in both cases, however, because solidification requires cooling through the glass transition in the first case, and cooling only to the solidus in the second.



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