THERMAL DIFFUSIVITY OF CALCIUM-RICH ROCKS

The ability of rocks to conduct heat exerts a fundamental control on many geologic processes, from the cooling of plutons to the deformation of continents. k (W/m/K) = DpC_P, where D is thermal diffusivity (m²/s), p is density (kg/m³) and C_P is heat capacity (J/kg/K). Thermal conductivity is typically assumed to be a constant for the purposes of numerical modeling, despite the fact that it is temperature-dependent and varies widely between different rock types. To some extent the increase in heat capacity offsets the decrease in thermal diffusivity towards higher temperatures, but overall thermal conductivity can still vary by a factor of two or more over the temperature range expected in typical continental crust. The laser-flash analysis method provides accurate measurements of D to high T, avoiding problems with spurious radiative transfer inherent in some other laboratory techniques.

We are measuring thermal diffusivity, density and heat capacity for a collection of calcium-rich igneous and metamorphic rocks in order to determine their thermal conductivity over a wide temperature range. We will explores the impact this may have on the geological setting in which they are found. The collection consists of: metamorphic calc-silicate gneisses, and igneous anorthosites and alkali carbonatites. Calc-silicates act as wall rocks to some plutons, and as such their thermal conductivity is a significant control on the cooling rate of these plutons. The thermal diffusivity of the anorthosite rocks are compared against those previously measured for plagioclase crystals in order to examine the effect of mineral grain boundaries on heat transfer. Carbonatites are often associated with U and Th, which produce radiogenic heat. Inefficient removal of this heat by conduction could lead to the formation of a local “hotspot”, which would affect crustal rheology and fluid flow.