Laboratory measurements of P- and S-wave velocities in polycrystalline plagioclase and gabbronorite up to 700 °C and 1 GPa:: Implications for the low velocity anomaly in the lower crust

P- (Vp) and S-wave (Vs) velocities of polycrystalline plagioclase (An(51+/-1)) and gabbronorite including plagioclase ( An(49 +/- 2)) ( 62.3vol.%) were measured up to 700 degrees C at 0.6, 0.8, and 1.0GPa during heating and cooling. Both polycrystalline plagioclase and gabbronorite show reversible and discontinuous change in temperature derivative of Vp and Vs, partial derivative Vp/partial derivative T and partial derivative Vs/partial derivative T, respectively, at about 400 degrees C. The partial derivative Vp/partial derivative T and partial derivative Vs/partial derivative T of the polycrystalline plagioclase changes from - 0.8 x 10(-4) to - 3.4 x 10(-4) km s(-1) degrees C(-1) and from - 1.1 x 10(-4) to - 3.3 x 10(-4) km s(-1) degrees C(-1), respectively. The partial derivative Vp/partial derivative T and partial derivative Vs/partial derivative T of the gabbronorite varies from - 1.0 x 10(-4) to - 3.4 x 10(-4) km s(-1) degrees C(-1) and from - 0.4 x 10(-4) to - 3.5 x 10(-4) km s(-1)degrees C(-1), respectively. The reversible and discontinuous changes in partial derivative Vp/partial derivative T and partial derivative Vs/partial derivative T are attributed to a phase transformation of plagioclase. The present data suggest that the sharp decrease in Vp and Vs would cause low velocity anomaly under dry and subsolidus melt-absent conditions in the mid-to-lower crust of relatively high heat flow regions.