Anomalous Shear Properties of Coesite at High Pressure and Implications for the X-discontinuity in the Earth's Upper Mantle
Abstract
The X-discontinuity (250-350km) in the upper mantle has been revealed under some continental or oceanic plates by a number of seismic studies, at which depth the P and S wave velocities increase by about 2%. One possible cause for this discontinuity, as suggested in previous studies, is the coesite-stishovite phase transition. Although the compressional behavior of coesite was determined by a previous single crystal study, its shear properties at high pressure have not yet been assessed experimentally. In this study, we conducted ultrasonic interferometry measurements on polycrystalline coesite up to 12.5GPa at ambient temperature. The sample was hot-pressed at 6.5GPa and 920°C and confirmed to be single phased by X-ray diffraction. We find that while the P wave velocities of coesite continuously increase with pressure, the S wave velocities exhibit a monotonic decrease to the peak pressure of the current experiment followed by a reversible recovery upon release of pressure. By fitting to finite strain equations, the elastic bulk and shear moduli and their pressure derivatives are precisely determined using the data collected during compression and decompression. Comparing with stishovite, coesite has lower pressure derivatives for both the bulk and shear moduli that is especially pronounced for the shear modulus. The volume-pressure relations obtained in the current study are in excellent agreement with those from single crystal compression of coesite. These results indicate that the velocity and impedance contrasts of the coesite-stishovite transition will increase with pressure, and its seismic signatures will be greatly enhanced at the depths of the X-discontinuity.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMMR43A2374C
- Keywords:
-
- 3924 MINERAL PHYSICS High-pressure behavior