Experimental dehydration kinetics of serpentinite using pore volumometry

A series of dehydration experiments was carried out on both intact rock and cold-pressed powdered samples of serpentinite at temperatures in the range 535-610 degrees C, 100-170 degrees C above the onset of the breakdown temperature of 435 degrees C. Pore water pressures near 120 MPa were servo-controlled using a pore volumometer that also allowed dehydration reaction progress to be monitored through measurement of the amount of evolved water. Effective hydrostatic confining pressures were varied between 0 and 113 MPa. The reaction rate of intact specimens of initially near-zero porosity was constant up to 50-80% reaction progress at any given temperature, but decreased progressively as transformation approached completion. Water expulsion rates were not substantially affected by elevation of effective pressures that remained insufficient to cause major pore collapse. An Arrhenius relation links reaction rate to temperature with an activation enthalpy of 429 +/- 201 and 521 +/- 52 kJ mol(-1) for powdered and intact specimens, respectively. Microstructural study of intact specimens showed extensive nucleation beginning at pre-existing cracks, veins and grain boundaries, and progressing into the interior of the lizardite grains. Extrapolation of these data towards equilibrium temperature provides an upper bound on the kinetics of this reaction in nature.