THERMAL DEHYDRATION REACTIONS CHARACTERIZED BY COMBINED MEASUREMENTS OF ELECTRICAL-CONDUCTIVITY AND ELASTIC-WAVE VELOCITIES

Combined laboratory measurements of seismic velocities and electrical conductivity as a function of PT and drainage conditions have been per formed on various rocks containing hydrous minerals. This paper presen ts experimental results for evaporite rocks containing gypsum (CaSO4 . 2H2O) and carnallite (KMgCl3 . 6H2O) and for serpentinite- The experi ments on the evaporite rocks were carried out in a triaxial cell in th e range 20-130-degrees-C and 5.0-24.0 MPa confining pressure. The meas urements on serpentinite covered the range 20-750-degrees-C at pressur es of up to 200 MPa and were performed in a cubic pressure apparatus. The thermally induced onset of dehydration was indicated by the pronou nced discontinuous behaviour of conductivity. corresponding to a marke d drop in the elastic wave velocities. The respective Poisson ratios w ere inversely correlated with permeability and seem to be a sensitive parameter to describe the reaction-related variations of pore space. T he seismic properties can be satisfactory modelled by using the self-c onsistent approximation of O'Connell and Budiansky [1], illustrating t he effects of variations in pore space and saturation. The dehydration behaviour of the various hydrous rock types was found to be different because the changes in the rock physical properties are closely linke d to the internally created pore fluid, to the changes in the porosity and pore geometry, and to the resulting pore pressure. Progressive th ermal dehydration reactions induce an opening of pore space accompanie d by lowered saturation conditions whereby the rate of fluid release d epends mainly on temperature and also on the drainage conditions of th e system. Our findings, as reflected by the variations in the geophysi cal parameters, may be of importance for the interpretation of natural dehydration processes in the crust caused by prograde metamorphism.