Temperature derivatives of elastic wave velocities in plagioclase (An51±1) above and below the order-disorder transition temperature

Compressional (v(p)) and shear (v(s)) wave velocities of plagioclase (An(51 +/- 1)) were measured up to 900 degrees C at 1 GPa. The temperature derivatives of nu(p) (partial derivative v(p)/partial derivative T) and nu(s) (partial derivative v(p)/partial derivative T) show a discontinuous change at similar to 400 degrees C. The partial derivative nu(p)/partial derivative T is -0.9 x 10(-4) km/s/degrees C below 400 degrees C and -4.4 x 10-(4) km/s/degrees C above 400 degrees C. The partial derivative nu(p)/partial derivative T also increases from -0.7 x 10(-4) to -4.1 x 10(-4) km/s/degrees C. These nu(p) and nu(s) show reversible changes between 25 and 700 degrees C. In contrast, both nu(p) and nu(s) increase (0.08 and 0.08 km/s, respectively) at 700-800 degrees C, and show irreversible changes after heating to 800 and 900 degrees C. The X-ray powder diffraction analysis shows that the run product heated to 900 degrees C shows a higher lattice angle gamma than the run products obtained on heating up to 700 'C, which is comparable to the lattice angle y of high and low plagioclase, respectively. We ascribe the discontinuous change in nu(p), nu(s), partial derivative nu(p)/partial derivative T, and partial derivative nu(s)/partial derivative T to the order-disorder transition of plagioclase at high temperatures.