A dehydroxylation kinetics study of brucite Mg(OH)2 at elevated pressure and temperature

Abstract

We performed an in situ dehydroxylation kinetics study of brucite under water-saturated conditions in the pressure and temperature ranges of 593–633 K and 668–1655 MPa using a hydrothermal diamond anvil cell. The kinetic analysis of the isothermal–isobaric data using an Avrami-type model involving nucleation and growth processes yields values for the dehydroxylation rate and reaction order compatible with a reaction mechanism limited by the monodimensional diffusion of water molecules from structural OH groups. Our results show a negative pressure dependence on the reaction rate k and a positive temperature dependence on the k. The dehydroxylation of brucite yields an activation volume ∆V value of 5.03 cm3/mol. Following the Arrhenius relationship, the apparent activation energy E a of the process is calculated to be 146 kJ/mol within the experimental P–T ranges. It is determined that the fluid production rates varying from 4.4 × 10−7 to 10.7 × 10−7 $${\text{m}}_{\text{fluid}}^{3} \;{\text{m}}_{\text{rock}}^{ - 3} \;{\text{s}}^{ - 1}$$ are a few orders of magnitude greater than the strain rate of the mantle serpentinites, which may be fast enough to result in the brittle fracture of rocks. Moreover, the rate of fluid production will be enhanced when brucite occurs in the non-/low H2O environments of the subduction zone.