Thermal diffusivity of clinopyroxenes at elevated temperature

Abstract: The phonon component of thermal diffusivity (D) was measured from 8 single-crystals and 2 polycrystalline clinopy-roxene samples at temperatures (T) up to a maximum of ∼ 1000 to 1600 K, using laser-flash analysis. Electron microprobe analysis shows that we have two samples of near end-member diopside, two augites, one end-member and one impure jadeite, aegerine, and three near end-member spodumenes. Hydroxyl contents determined from near-IR spectroscopy range from ∼ 0 to 70 ppm as H2O. Two directions constrain D for clinopyroxenes, consistent with phonon symmetry. Anisotropy = (D⊥c−D||c)/D||c is near 40 % for all samples and independent of T, where | | and ⊥ indicate the direction heat flows relative to the c-axis. Thermal diffusivity decreases with increasing T and approaches a constant (Dsat) near 1400 K. The temperature dependence of 1/D is well described by low-order polynomial fits. At 298 K and for near end-member compositions, cation mass and bond lengths strongly affect D⊥c whereas D||c is essentially constant and large, 3.9 mm2 s−1. For solid solutions, D at 298 K decreases from end-member values as impurity content increases, but D at saturation is little affected. Excluding Li-rich clinopyroxenes, D⊥c,sat ∼ 0.5 mm2 s−1 and D||c,sat ∼ 0.9 mm2 s−1. The values pertain to lithospheric and mantle compositions of clinopyroxene above 1400 K, and lead to bulk thermal conductivity at saturation being 2.4 W m−1 K−1. Pressure deriviatives are predicted from models as ∂(ln k)/∂P = 4.2 to 4.7 %GPa−1. Key-words: Laser-flash method, high-temperature, thermal diffusivity, thermal conductivity, IR spectroscopy, clinopyroxene