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

Abstract

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 °C under H 2O-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 °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 H 2O 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.