THERMALLY INSULATING PARTIALLY MOLTEN ZONES IN PLANETARY MANTLES PROMOTE MELT PRODUCTION DURING DECOMPRESSION

There have been relatively few measurements of the thermal properties of rocks, minerals and melts to high temperature, despite the importance of these properties for modeling planetary interiors and understanding planetary evolution. We measured thermal diffusivity (D), heat capacity (C_P), density (ρ) and viscosity (η) for 12 remelted natural lavas and 4 synthetic glasses and melts, ranging in composition from leucogranite to low-silica basalt, and calculated their thermal conductivity (k = DρC_P). Both viscosity and the glass transition temperature decrease with decreasing melt polymerization. For basaltic glasses, D is low, ~0.5 mm²s^-1 at room temperature, decreases slightly with increasing temperature, and then drops upon melting to ~0.25 to 0.35 mm²s^-1. Other samples behave similarly. Despite scatter, clear correlations exist between D of glass or melt with Si content, density, NBO/T, and, most strongly, with fragility (m), which represents the degree to which melt viscosity is non-Arrhenian. Glass thermal diffusivity is represented by D = FT ^–G +HT, where F, G and H are fitting parameters. For melts, ∂D/∂T was resolved only for dacite-andesite and MORB: a positive slope is consistent with other iron-bearing samples. Glass and liquid C_P depend on density and other physical properties, but not exactly in the same manner as D. We calculate thermal conductivity (k) from these data and demonstrate that k for glasses is described by a Meier-Kelly formula. Large scatter exists for k at 298 K, but silicic to intermediate melts have k between 1.8 and 1.3 Wm^-1K^-1, whereas basaltic melts are constrained to ~1.4 ± 0.1 Wm^-1k^-1.

Low values for thermal diffusivity and viscosity for basaltic melts suggests that basalts transfer heat much more efficiently by advection than by conduction alone, and that partially molten zones in the mantle quickly become more thermally insulating than non-molten zones, potentially contributing to melt localization during decompression melting.