Metamorphic reaction kinetics at “dry” and “wet” conditions in the binary MgO-SiO2 system

Reaction rims contain a wealth of information that can be used to decipher the P-T-t-X history of metamorphic and metasomatic rocks. One of the most important parameters that controls reaction rim growth dynamics is the presence of volatiles, which can affect rim thicknesses, phase stabilities and the development of rim microstructures.

We performed reaction rim growth experiments to investigate net-transfer reactions in both “dry” and “wet” systems. Reaction rims were produced between single crystals of quartz and periclase in an internally heated pressure vessel (IHPV) at 0.3-0.4 GPa and 1100-1300 °C for 66 h to 168 h. In “wet” experiments, a ppm amount of water was introduced by pre-annealing periclase single crystals at water saturated conditions, which is gradually released during rim growth experiments.

First results indicate the formation of single enstatite or forsterite and double enstatite-forsterite reaction rims in the “dry” and “wet” systems respectively. When water-doped periclase is used as a reactant, reaction rim growth rates increase from 10-18.23±0.18 to 10-17.67±0.03 m2/s at 1100 °C and from 10-16.95±0.06 to 10-16.16±0.01 m2/s at 1200 °C. In experiments that were entirely anhydrous, no reaction rim formed at all. These observations suggest that water may act as a catalyst and that a certain fluid threshold is needed to initiate metamorphic re-equilibration.

These findings imply that reaction rims may be used to discriminate between kinetically “dry” and “wet” systems and have thus the potential to monitor the presence of extremely small amounts of water during metamorphic reactions in natural systems.