Determination of reaction kinetics using grain size: An application for metamorphic zircon growth

Abstract

The reaction kinetics of metamorphic minerals can be subdivided into interface- and diffusion-controlled kinetics. The discrimination of reaction kinetics is crucial for estimating reaction rates. Here, we propose a new and simple method for discriminating reaction kinetics. This method requires measuring only the initial and final grain sizes during growth. The reaction kinetics is inferred from different plotted arrays of initial vs. final grain sizes after the mineral growth. Using metamorphic zircon, we take detrital core sizes as the initial sizes and post-metamorphic grain sizes as the final sizes. The application of the method to the subduction-related high-pressure Nagasaki metamorphic complex in Japan shows that this metamorphic zircon grew under interface-controlled kinetics even at the relatively low temperature of 440°C. This method is potentially applicable to other minerals that have time-markers, such as chemical zoning or internal structures that are captured at a given point in time during growth.