Ultra-fast metamorphic reaction during regional metamorphism

Abstract

Constraining the timescales of metamorphic processes is critical to understanding geodynamics on Earth. It is generally accepted that the rates of metamorphic reactions in regional metamorphism, where fluids are limited or transient, are several orders of magnitude slower than in laboratory experiments. This discrepancy is attributed to several rate-limiting mechanisms affecting metamorphic reactions in natural settings, such as differences in the reactive surface area of the reactants, the magnitude of the driving force for reaction, rates of inter-granular transport and possible fluid content. Here we report an ultra-fast metamorphic reaction within a year, constrained by diffusion modeling on frozen-in chemical gradients of trace elements preserved in metamorphic garnet across a partially melted corona texture. The growth of peritectic garnet occurred in the presence of a melt phase, which distributed along the grain boundaries. The observed chemical gradient of HREE+Y in garnets is interpreted to have formed due to trace element diffusion in the inter-granular melt, recorded by the simultaneous growth of multiple garnet grains across the corona texture. A diffusion model using a fixed boundary condition suggests a timescale of 8.4 (+5.4/-3.3) days for the formation of this corona texture, whereas a moving boundary model provides a slightly longer timescale of less than a year. These timescales are much shorter than those previously obtained from regional metamorphism in nature, but are similar to contact metamorphism in nature and laboratory-based results. Based on these findings, we propose that ultra-fast pulses of metamorphic reactions occur in nature under fluid/melt-present conditions, as elemental diffusion and mass transport in an aqueous fluid or melt are significantly faster than those in mineral lattices and anhydrous grain boundaries. However, rapid metamorphic reactions are difficult to identify due to the insufficient temporal resolution of radioisotope dating and the poor preservation of chemical gradients during subsequent metamorphic reactions.