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

Abstract

The thermal evolution of planetary crust and lithosphere is largely governed by the rate of heat transfer by conduction^1,2,3. The governing physical properties are thermal diffusivity (κ) and conductivity (k = κρC_P), where ρ denotes density and C_P denotes specific heat capacity at constant pressure. Although for crustal rocks both κ and k decrease above ambient temperature^4,5, 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 analysis^8,9 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 C_P 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 C_P 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.