THE DEPENDENCE OF ROCK THERMAL PROPERTIES ON COMPOSITION AND TEMPERATURE

Modeling geothermal energy resources requires knowledge of rock thermal properties, including thermal diffusivity (D), heat capacity (C_P) and thermal conductivity (k = DρC_P, where ρ is density), because heat flow is proportional to thermal conductivity and to the geothermal gradient (Fourier's Law). Each of these properties varies with mineralogical composition, and also with temperature. Density varies relatively little as a function of T, and can be accurately calculated if the mineralogy is known. Heat capacity increases with increasing T, but varies relatively little between different rocks types and can also be accurately calculated if the mineralogy is known. Thermal diffusivity is more complex, varying widely at as a function of mineralogy and temperature (e.g. D of major crustal rock types varies by a factor of ~4 at ambient temperatures, and typically decreases by a factor of 2 to 5 over the range 20 to 500˚C). Many previous results contain systematic errors associated with imperfect contacts and ballistic radiative transfer (heat transport by radiation that does not interact with or warm the sample, and is directly generated by the heating element). Using Laser Flash Analysis (LFA), which limits these systematic errors, we have measured the temperature-dependent thermal diffusivity of major rock types across relevant lithospheric temperatures.

Examples of our results and applications include (i) the crustal geotherm is straighter than calculated with constant thermal properties, because higher (average) radiogenic heat production in the upper crust is offset by higher (average) thermal conductivity at low temperatures. (ii) Dolomitized basins have higher thermal conductivity than calcite-dominated basins, and may have deeper maturation windows. (iii) including temperature-dependent thermal properties of country rocks typically leads to increases in solidification time for igneous intrusions of a factor of ~2. (iv) Intrusions can have long-lived non-magmatic thermal effects due to their different radiogenic heat production and thermal conductivity relative to country rocks.