CROSS PRESSURE AND TEMPERATURE DERIVATIVES OF SELECTED ELASTIC-MODULIFOR OLIVINE FROM GIGAHERTZ ULTRASONIC INTERFEROMETRY

Techniques have been developed to measure the cross pressure and tempe rature dependence of elastic moduli of olivine using the gigahertz ult rasonic interferometric method in a gas pressure vessel (to 500 MPa) e quipped with an internal heater (to 500 K), Special attention has been drawn to the effect of the bond which couples the sample to the buffe r rod. We obtained partial derivative(2)C(22)/partial derivative P par tial derivative T = (2.8 +/- 0.9) x 10(-3) K-1 for a San Carlos olivin e sample with a lapped contact. Selective values of partial derivative (2)K(T)/partial derivative P partial derivative T (0, 0.5 x 10(-3), 1. 0 x 10(-3) K-1) are used to calculate their effect on cell volume, bul k modulus, and bulk sound velocity for olivine at pressures to 14 GPa and temperatures to 1800 K (410-km depth), using the high-temperature form of the Birch-Murnaghan equation of state. The largest difference between the cell volumes for partial derivative(2)K(T)/partial derivat ive P partial derivative T = 1 x 10(-3) K-1 and for partial derivative (2)K(T)/partial derivative P partial derivative T = 0 is about 0.6% at the highest pressure and the highest temperature (14 GPa, 1800 K). Th erefore PVT data with precision at least better than 0.6% are required to distinguish between the extreme values of partial derivative(2)K(T )/partial derivative P partial derivative T for olivine if the pressur e and temperature data are free of errors. If the cross pressure and t emperature derivative term is neglected (partial derivative(2)K(T)/par tial derivative P partial derivative T = 0) in extrapolating laborator y elasticity data to mantle pressure and temperature conditions, the b ulk modulus of olivine can be underestimated by as much as 10% and the bulk sound velocity by 3% at the 410-km seismic discontinuity. Thus t he present study highlights the need to measure elastic properties of mantle candidate minerals at the P-T conditions of the mantle transiti on zone in order to establish a more precise model of the mantle miner alogy.