P and S wave velocities of deep crustal rocks under high pressure and high temperature conditions

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We carry out research on the elastic properties of the crustal rocks using ultrasonic measurements. We have developed techniques to perform ultrasonic measurements at mid-to-lower crustal conditions of pressure and temperature. These techniques are now been applied to study the major crustal rocks, including gabbro, tonalite, granite, anorthosite, granulite and amphibolite, which were collected from (1)island arc deep crustal sections of Tanzawa Mountain of central Japan and Kohistan area of Pakistan, (2)Ichinomegata of NE Japan, Takashima and Kurose of SW Japan, and (3)Archean ultra-high temperature granulite-facies complex (Napier Complex) and Pan-African erogenic belt (Lutzow-Holm Complex) of East Antarctica. In order to interpret seismic structure of island arcs NE Honshu arc, the Vp values measured at lower temperatures than those of mid-to-lower crusts were compared with the seismic velocity profiles which were adjusted to value at 25 □ by following the temperature derivative giv … More en by Rudnick and Fountain (1995) and employing the geothermal gradient (Nishimoto et al.,2005). In order to compare directly to seismic velocities at the deep island arc pressures and temperatures, we developed ultrasonic velocity measurements using buffer rod technique. The measurements have doubled the temperature range of previous ultrasonic measurements on rocks. Pt buffer rod is used to isolate the piezoelectric transducer from the high-temperature condition. Travel times through the rock sample were determined with the pulse reflection technique. Kono et al.(2004a) measured compressional wave velocities (Vp) of gabbronorite, pyroxene granulite, wehrlite and websterite up to 1000□ at 1 GPa and found discontinuous change in temperature derivative of Vp in lower crustal rocks arc This reversible and discontinuous change is difficult to explain by the previously proposed mechanisms, such as thermal cracking, dehydration reaction, and/or partial melting. The data suggest that this reversible and discontinuous change would be associated with phase transition in plagioclase (Kono et al.2004b). Less