Thermal diffusivity and thermal conductivity of alkali feldspar at 0.8–3 GPa and 300–873 K

The heat transport properties of feldspar, one of the major minerals of the crust, are important for constraining the thermal state of the Earth’s crust. The thermal diffusivity (D) and thermal conductivity (κ) of two natural alkali feldspars, namely, perthite and albite (Ab), were simultaneously measured at high temperatures (300–873 K) and high pressures (0.8–3 GPa) using a transient plane-source method. The present results show that the D and κ of these alkali feldspars decreased with the increase in temperature, whereas the κ of perthite remained almost constant at above 450 K. The D and κ of these samples decreased by 24–35% and 8–21% when the temperature increased from 300 to 873 K, respectively, suggesting that phonon conduction may be the dominant mechanism. The D and κ of these samples also exhibited a positive pressure dependence as indicated by their positive pressure coefficients: 0.052–0.098 mm2s−1GPa−1 for D and 0.189–0.325 Wm−1K−1GPa−1 for κ. Combining previous data with the results of this study, the D and κ of an intermediate albite-orthoclase solid solution can be reasonably estimated by an empirical model. Furthermore, the average κ of the crust was recalculated to accurately constrain the thermal thickness and temperature of the lithosphere. The present estimate suggests that partial melting can occur at shallow depths of the middle and lower crust, which may provide a new understanding of the low-velocity and high-conductivity anomalies revealed by geophysical observations in the crust.