Temperature-dependent thermal diffusivity of the Earth’s crust and implications for magmatism

The rate of heat transfer by conduction is the dominant factor that determines the thermal evolution of planetary crust and lithosphere. Most thermal models of the Earth's crust assume constant values for thermal diffusivity, owing to large experimental uncertainties in measuring this property of rocks at high temperature. Whittington et al. have used recent advances in laser-flash analysis on three different crustal rocks types to show that thermal diffusivity strongly decreases with increasing temperature. They find thermal diffusivity to be about half that commonly assumed at mid-crustal temperatures and therefore conclude that the hot middle and lower crust is a much more effective thermal insulator than previously thought. They also present models of lithospheric thermal evolution during continental collision, and demonstrate that the temperature dependence of rock properties leads to a positive feedback between strain heating in shear zones and more efficient thermal insulation, removing the requirement for unusually high radiogenic heat production to achieve crustal melting temperatures. The thermal evolution of planetary crust and lithosphere is governed by the rate of heat transfer by conduction, which is determined by the rock's thermal diffusivity, usually assumed to remain constant. Alan Whittington and colleagues show that thermal diffusivity in fact decreases strongly with increasing temperature, concluding that the hot middle and lower crust is a much more effective thermal insulator than previously thought; this removes the requirement for unusually high radiogenic heat production to achieve crustal melting temperatures. The thermal evolution of planetary crust and lithosphere is largely governed by the rate of heat transfer by conduction1,2,3. The governing physical properties are thermal diffusivity (κ) and conductivity (k = κρCP), where ρ denotes density and CP denotes specific heat capacity at constant pressure. Although for crustal rocks both κ and k decrease above ambient temperature4,5, most thermal models of the Earth’s lithosphere assume constant values for κ (∼1 mm2 s-1) and/or k (∼3 to 5 W m-1 K-1)6,7 owing to the large experimental uncertainties associated with conventional contact methods at high temperatures. Recent advances in laser-flash analysis8,9 permit accurate (±2 per cent) measurements on minerals and rocks to geologically relevant temperatures10. Here we provide data from laser-flash analysis for three different crustal rock types, showing that κ strongly decreases from 1.5–2.5 mm2 s-1 at ambient conditions, approaching 0.5 mm2 s-1 at mid-crustal temperatures. The latter value is approximately half that commonly assumed, and hot middle to lower crust is therefore a much more effective thermal insulator than previously thought. Above the quartz α–β phase transition, crustal κ is nearly independent of temperature, and similar to that of mantle materials11. Calculated values of k indicate that its negative dependence on temperature is smaller than that of κ, owing to the increase of CP with increasing temperature, but k also diminishes by 50 per cent from the surface to the quartz α–β transition. We present models of lithospheric thermal evolution during continental collision and demonstrate that the temperature dependence of κ and CP leads to positive feedback between strain heating in shear zones and more efficient thermal insulation, removing the requirement for unusually high radiogenic heat production to achieve crustal melting temperatures. Positive feedback between heating, increased thermal insulation and partial melting is predicted to occur in many tectonic settings, and in both the crust and the mantle, facilitating crustal reworking and planetary differentiation12.