The influence of OH in coesite on the kinetics of the coesite-quartz phase transition

Coesite is an important pressure indicator for metamorphic rocks of ultra-high pressure origin. However, it often does not survive exhumation, but reacts back to quartz. Although it was shown experimentally that hydrogen incorporation in coesite increases with increasing pressure, the few coesite relics, which are found in nature, are “dry”. Thus, does the incorporation of hydrogen promote the back-reaction of coesite to quartz during exhumation? To investigate the influence of intrinsic OH in coesite on the kinetics we investigated the coesite quartz phase transformation by using “dry” coesite with 60 H / 10 Si and “wet” coesite with 700 H / 10 Si. Dry coesites were synthesized in a piston-cylinder device apparatus from fine grained quartz powder at 4.0 GPa and 800 ± 5 °C. Hydrous coesite samples were synthesized in multi-anvil apparatus from highly pure SiO2 with water in excess at 7.5 GPa and 1100 ± 5 °C. The water contents of the starting coesites were quantified by IR spectroscopy [1]. Although we worked hard to avoid OH incorporation, the “dry” coesite still contents 60 ± 10 H / 10 Si. The hydrogen coesite of the “wet” coesite sample was 700 ± 100 H / 10 Si. The multi-anvil type X-ray high-pressure/high-temperature system, MAX80 was used for our kinetic studies. In the first series of experiments the transformation kinetic of “dry” coesite was determined in the quartz stability field in the pressure range 2.4-3.0 GPa. The duration of the experiments varies from 60 to 120 min after reaching given pressure and temperature. The counting time for the diffraction pattern was 120 seconds. The transformation kinetic of the “wet” coesite was determined under the same conditions. Counting time for these diffraction patterns was 30–120 seconds. Diffraction patterns were recorded in an energy dispersive mode using white synchrotron X-rays from the storage ring DORIS III. The integrated areas for representative diffraction peaks were obtained by fitting each individual pattern. The peak intensities of the (040) diffraction peak of coesite and (101) diffraction peak of quartz were used to determine the degree of transformation X. Figure1 illustrates time-resolved diffraction patterns for the phase transformation of “dry” coesite to quartz.