The effect of hydration and dehydration reactions on wave velocities in basalts

Compressional and shear wave velocities were measured on zeolite-bearing an d zeolite-free basalts at pressures up to 600 MPa and at temperatures up to 900 degrees C. Measurements were done under dry and wet conditions in a mu lti-anvil pressure apparatus and in an internally heated gas apparatus at d ifferent effective pressures (P-eff): (1) with atmospheric pore pressure (P -eff = P-conf), and (2) at pore pressure equal to confining pressure (P-eff = 0), respectively. Temperature-induced abrupt velocity variations were fo und to be caused by hydration and dehydration processes. At conditions of z ero effective pressure, the formation of zeolite gives rise to an increase of wave velocities at around 200 degrees C in the zeolite-free basalt due t o filling of cracks and fissures by the newly formed minerals and the coeva l enhancement of grain-boundary contacts. Dehydration reactions in some zeo lite-bearing tholeiitic basalts at initially high effective pressures cause a significant abrupt and irreversible drop of P- and S-wave velocities at temperatures around 350 degrees C in the initially dry rock. The declines o f velocity are attributed to spontaneous lowering of effective pressure due to internally created pore pressure that induces the opening of pore space s and reconstitution of pore geometry. (C) 1999 Elsevier Science B.V. All r ights reserved.