ABSTRACT
A review on the recent progress of our ultrasonic velocity measurements at simultaneous high-temperature and high-pressure conditions is given. Using the pulse transmission technique, the compressional ultrasonic velocity (Vp) of various rocks at simultaneous high-pressure and high-temperature conditions up to 5.5GPa and 1500°C has been measured. According to our experimental results, in a microcrack-free sample, the observed compressional velocity (Vp) will be sharply reduced by a large amount upon initial compression. This effect gradually decreases with pressure and totally disappears at pressures above 2.5 GPa. Accordingly at ambient conditions, the Vp in a basalt sample without microcracks should be 6.856 km/s, much higher than the value, measured from the sample with microcracks, of 6.044 km/s which was the normal accepted value in handbooks. Hence we believe that microcracks exist in most of basalt samples which have been tested ever.
In the simultaneous high-temperature and high-pressure experiments, a softening phenomenon occurs at a special temperature θx which indicates a kind of softening for rocks. It is noteworthy that the θx corresponds to the glass transition temperature θg for glass samples. Therefore the rocks behave similarly as glass at high temperature. In addition, basalt was found to transfer to eclogite above 3.5 GPa and 500°C which might be a major process between the subduction zone and mantle in the deep interior of the Earth.
