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

Compressional and shear wave velocities were measured on zeolite-bearing and 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 multi-anvil pressure apparatus and in an internally heated gas apparatus at different 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 found to be caused by hydration and dehydration processes. At conditions of zero 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 to filling of cracks and fissures by the newly formed minerals and the coeval enhancement of grain-boundary contacts. Dehydration reactions in some zeolite-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 of velocity are attributed to spontaneous lowering of effective pressure due to internally created pore pressure that induces the opening of pore spaces and reconstitution of pore geometry. (C) 1999 Elsevier Science B.V. All rights reserved.