Metamorphic reaction kinetics at anhydrous to water-saturated 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 is the presence of volatiles, which can affect rim thicknesses, phase stabilities and the development of rim microstructures. In this study, reaction rim growth experiments were performed between periclase and quartz at anhydrous to water-saturated conditions at 3-4 kbar and 1100-1300(degrees)C. Controlled minute amounts of water were added through OH-doped periclase, which enabled us to perform experiments at controlled water-undersaturated conditions. At anhydrous conditions, no reaction rim formed at all implying that water acts as a catalyst, and a minimum fluid threshold is needed to initiate metamorphic reactions. At water-undersaturated conditions extremely small variations in water content are sufficient to change reaction rim growth rates by multiple orders of magnitude. This implies that reaction rims have the potential to monitor variations in the amount of water at those grain boundaries that serve as fast pathways for component transport at water-undersaturated conditions during metamorphic and metasomatic reactions in natural systems, allowing them to be used as sensitive geohygrometers. Additionally, the effect of water on relative layer thicknesses may provide an application for reaction rim microstructures to be used as new physico-chemical gauges that will allow us to discriminate between water-undersaturated and water-saturated conditions during metamorphic events.

Automated summary

Reaction rims in metamorphic and metasomatic rocks contain important information that can be used to understand their history. This study focused on the influence of volatiles, particularly water, on reaction rim growth. The experiments showed that water acts as a catalyst and a minimum fluid threshold is needed for the initiation of metamorphic reactions. Reaction rims can be used as sensitive geohygrometers to monitor variations in water content at grain boundaries. Additionally, the effect of water on relative layer thicknesses can be utilized as a new physico-chemical gauge to distinguish between water-undersaturated and water-saturated conditions during metamorphic events.