Physics
Scientific paper
Oct 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994pepi...86...35r&link_type=abstract
Physics of the Earth and Planetary Interiors (ISSN 0031-9201), vol. 86, no. 1-3, p. 35-44
Physics
30
Compression Waves, Earth Mantle, Garnets, Polycrystals, Pressure Dependence, Propagation Velocity, S Waves, Seismic Waves, Bulk Modulus, Crystal Structure, Elastic Properties, Interferometry
Scientific paper
The pressure dependence of the elastic wave velocities for a polycrystalline specimen (100% theoretical density of majorite-pyrope garnet (Py62Mj38)) has been determined at room temperature to 3 GPa by ultrasonic phase comparison interferometry. Pressure derivatives of the adiabatic bulk modulus (dK(sub S)/dP=5.3) and shear modulus (dG/dP=2.0) are derived from the travel times of the compressional (P) and shear (S) waves. These derivatives clearly demonstrate that pressure has a more pronounced effect on the elasticity of the majorite-rich garnet than on the pure pyrope end-member, presumably in response to the coupled substitution of Si and Mg for Al on the octahedral site in the garnet structure. When combined with plausible values for the (as yet unmeasured) temperature derivatives, these new data can be used to calculate velocity-depth profiles along a 1400 C adiabat for the range of depths in which majorite is expected to be stable in the transition zone of the Earth's mantle (450-600 km). For shear waves, these majorite gradients match those from regional seismic models much better than those for a pure olivine mantle. More reliable calculations of elastic wave velocities at mantle temperatures and pressures await measurement of the temperature derivatives of elastic modulus.
Gwanmesia Gabriel D.
Liebermann Robert C.
Rigden Sally M.
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