Temperature-induced densification in compressed basaltic glass revealed by in-situ ultrasonic measurements

15 Acoustic velocities of a model basalt glass (64 mol% CaMgSi 2 O 6 + 36 mol% CaAl 2 Si 2 O 8 ) 16 were measured along different pressure (P)-temperature (T) paths. One set of experiments 17 involved isothermal compression-decompression cycles, performed at temperatures of 300 K, 18 641 K, 823 K, and 1006 K and pressures up to 12.2 GPa. The other set of experiments involved 19 constant-load heating-cooling cycles at temperatures up to 823 K and pressures up to 7.5 GPa. 20 Both sets of experiments were performed in a multi-anvil apparatus using a synchrotron-based 21 ultrasonic technique. Our results show that the glass compressed isothermally at 300 K (cold- 22 compression) displays anomalously decreasing compressional (V P ) and shear (V S ) wave 23