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Temperature derivatives of elastic wave velocities in plagioclase (An_51±1) above and below the order-disorder transition temperature

¹ Graduate School of Environment and Information Sciences, Yokohama National University, Yokohama 2408501, Japan
² Department of Geology and Mineralogy, Kyoto University, Kyoto 6068502, Japan

Compressional (v_p) and shear (v_s) wave velocities of plagioclase (An_51±1) were measured up to 900 °C at 1 GPa. The temperature derivatives of v_p (v_p/T) and v_s (v_p/T) show a discontinuous change at ~400 °C. The v_p/T is –0.9 x 10^–4 km/s/°C below 400 °C and –4.4 x 10^–4 km/s/°C above 400 °C. The v_p/T also increases from –0.7 x 10^–4 to –4.1 x 10^–4 km/s/°C. These v_p and v_s show reversible changes between 25 and 700 °C. In contrast, both v_p and v_s increase (0.08 and 0.08 km/s, respectively) at 700–800 °C, and show irreversible changes after heating to 800 and 900 °C. The X-ray powder diffraction analysis shows that the run product heated to 900 °C shows a higher lattice angle than the run products obtained on heating up to 700 °C, which is comparable to the lattice angle of high and low plagioclase, respectively. We ascribe the discontinuous change in v_p, v_s, v_p/T, and v_s/T to the order-disorder transition of plagioclase at high temperatures.

Key Words: Elastic wave velocity • plagioclase • phase transition • high temperature