Strong decrease in ultrasonic Vp in lower crustal rocks at high temperature

In order to determine temperature derivatives of P-wave velocities (Vp) at pressure-temperature conditions of island arc lower crust, we measured Vp in various lower crustal rocks from exposed crustal section of Kohistan arc at temperature up to 1000 ° C at pressure 1.0 GPa. The rock sample is embedded in a cylinder of pressure transmitting material (talc). Pt buffer rod is used to isolate the piezo-electric transducer from the high-temperature condition. Travel times of P-wave through the rock sample were determined with the pulse reflection technique. Sample length at experimental conditions was estimated with adiabatic bulk modulus and thermal expansion coefficient of rock samples studied. The uncertainties of Vp are +/- 0.3 %. Kohistan arc lower crust is mainly composed of gabbronorite, wehrlite (Chilas complex), amphibolite, tonalite (Kamila amphibolite), pyroxene granulite, garnet granulite, garnet pyroxenite, websterite, and dunite (Jijal complex). Vp measurements were performed for 6 rock samples at 100 ° C intervals up to 1000 ° C. Ultrasonic determinations of Vp in the Kohistan lower crustal rocks show nonlinear temperature dependence. With increasing temperature, Vp values decrease slightly at lower temperature conditions, and decrease rapidly at higher temperature conditions. Cusp on the Vp - temperature profiles was identified for all samples. Cusp locates at various temperatures (300-800 ° C) depending the rock sample measured. The temperature derivative above the cusp temperature is 1.8 - 5.8 times higher than the temperature derivative below the cusp temperature. Vp value is 7.0 km/s at 800 ° C and 1 GPa for the gabbronorite, 7.6 km/s at 800 ° C for the wehrlite, 7.2 km/s at 900 ° C for the garnet granulite, 7.5 km/s at 900 ° C for the garnet pyroxenite, 7.2 km/s at 900 ° C for the websterite, and 8.0 km/s at 900 ° C for the duntie.