Discontinuous change in temperature derivative of Vp in lower crustal rocks

Compressional wave velocities (Vp) of gabbronorite, pyroxene granulite, wehrlite and websterite were measured up to 1000 degreesC at 1 GPa during heating and cooling. The wehrlite and websterite, which contain secondary hydrous minerals, show irreversible change in temperature derivative of Vp (partial derivativeVp/partial derivativeT). During heating, these rocks show sudden decrease in Vp when dehydration reactions took place. During cooling, the rocks show a linear increase in Vp. In contrast, the gabbronorite and pyroxene granulite, which are nearly dry crystalline assemblage, show reversible and discontinuous change in partial derivativeVp/partial derivativeT at around 400 degreesC. It changes from - 2.4 and - 2.1 x 10(-4) km s(-1) degrees C-1 to - 4.7 and - 4.5 x 10(-4) km s(-1) degrees C-1. This reversible and discontinuous change in partial derivativeVp/partial derivativeT 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.