KINETICS OF THE COESITE-QUARTZ TRANSITION: APPLICATION TO THE EXHUMATION OF ULTRAHIGH-PRESSURE ROCKS

Abstract

The kinetics of the quartz–coesite phase transition has been studied in situ by X-ray diffraction in the 2·1–3·2 GPa, 500–1010°C pressure–temperature range. Analysis of the data within Cahn's model of nucleation and growth at grain boundaries reveals that the prograde and retrograde reactions have different kinetics. The quartz → coesite transformation is one order of magnitude faster than coesite → quartz. Both reactions are characterized by high nucleation rates, so that the overall reaction kinetics is controlled by crystal growth processes. For the coesite → quartz transformation, growth rates are extrapolated using Turnbull's equation with an activation energy for the transition of 163 ± 23 kJ/mol. This kinetic law is combined with an ‘inclusion in a host’ elastic model to study the contribution of kinetics in coesite preservation. This numerical modelling shows that above 400°C retrograde transformation of coesite to quartz is mainly controlled by the ‘pressure vessel’ effect of the host phase, whereas reaction kinetics is the controlling factor at lower temperatures. The influence of the shape of the P – T path and the exhumation rate upon the retrogression of coesite to quartz are investigated to use the percentage of unretrogressed coesite inclusions to constrain P – T – t paths.