Springer Online Journal Archives 1860-2000
Chemistry and Pharmacology
Abstract The thermal diffusivity tensors at ambient pressure and temperature of three silicate mineral phases abundant in the upper mantle (San Carlos olivine [Mg0.89Fe0.11]2SiO4, Kilbourne Hole orthopyroxene [Mg1.63Fe0.17Ca0.04Mn0.01] [Cr0.01 Al0.12] [Si1.89Al0.11]O6 and a garnet of intermediate composition Py51Al32Gr16Sp1 are reported. The extension to high pressure and temperature of the experimental technique employed here is discussed and, for olivine, data at high pressure are also reported. The diffusivity in the two orthorhombic minerals is highly anisotropic, the components of the tensor along the a, b, and c crystallographic axes, in units of mm2/sec, being [2.16 1.25 1.87] in the case of olivine and [1.26 1.05 1.66] in the case of the orthopyroxene. The isotropic thermal diffusivity in garnet is 1.06 mm2/ sec. The experimental uncertainty is approximately 2%. The pressure dependence of thermal diffusivity is approximately 4% per GPa. The relation of thermal to elastic anisotropy is briefly considered. A model incorporating elastic anisotropy, anharmonicity described by acoustic Grüneisen parameters, Brillouin zone structure, and the increased phase volume for the scattering of short wavelength phonons provides a qualitatively reasonable description of the thermal diffusivity anisotropy. Since both olivine and orthopyroxene are aligned by flow deformation processes, the upper mantle is expected to be thermally anisotropic.
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