Kinetics of antigorite dehydration: A real-time X-ray diffraction study

Antigorite dehydration has been studied in situ using time-resolved X-ray diffraction (XRD) in the 1.1-5.5 GPa pressure range. The sequence of XRD spectra reveals that: the high-temperature stability limit of antigorite is decreased by 50-100 degrees C under H2O-unsaturated conditions. The decomposition of antigorite into the high-temperature products, forsterite+enstatite, proceeds via an intermediate assemblage of forsterite and a talc-like phase, observed within a temperature interval of 130 +/- 20 degrees C. The analysis of the transformation-time data using the Avrami model suggests that the breakdown of antigorite and the talc-like phase is kinetically controlled by surface growth processes at the edges of grains. The overall transformation rates are 10 to 100 times faster than those observed in water-saturated experiments, and show that the H2O activity is a strong driving force for dehydration. The breakdown of antigorite in subducting oceanic mantle with such kinetics would result in a fluid discharge rate of the order of 10(-6) to 10(-8) s(-1). This is faster than the viscous relaxation of serpentinites, and could lead to brittle failure or weakening of pre-existing faults. This provides an explanation for the seismicity in the lower plane of double seismic zones. (c) 2005 Elsevier B.V. All rights reserved.