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Thermal diffusivity of clinopyroxenes at elevated temperature

¹ Department of Earth and Planetary Sciences, Washington University, 1 Brookings Drive, St. Louis, Missouri 63130-4899, USA
² Department of Earth Science, Rice University, Houston, TX 77005, USA

^* Corresponding author, e-mail: hofmeist{at}levee.wustl.edu

The phonon component of thermal diffusivity (D) was measured from 8 single-crystals and 2 polycrystalline clinopyroxene 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 (D_sat) 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.