Garnet growth in a geological blink of an eye

Mineral reactions determine the physical and rheological properties of rocks, but whether these reactions occur close to or far from equilibrium and whether they are continuous or pulsed is challenging to unravel. This introduces significant uncertainty in determining the thermomechanical properties and behavior of the crust and estimating the pressure and temperature conditions that rocks underwent during their tectonic history. Here, we employ elemental mapping and high-precision Lu-Hf chronology to investigate whether and to what extent garnet—one of the most important recorders of pressure, temperature, deformation, and time in the lithosphere—keeps up with tectonic processes. The analysis was done on a single 1.2-cm-sized garnet grain from a carbonate-rich mica schist from the Schneeberg Complex (Italy). Five compositionally distinct zones were identified and dated separately. The four inner zones, characterized by trace-elements oscillations, yielded identical ages with a weighted mean of 98.4 ± 0.1 Ma (2σ), whereas the outermost zone yielded 97.8 ± 0.3 Ma. During the first growth pulse, garnet grew at an average radial growth rate of at least 6.2 cm m.y.⁻¹. Nucleation initiated out of equilibrium conditions and resulted in high fluid production that, in turn, boosted garnet growth, episodically limited by the rock’s elements transport permeabilities. This pulsed, ultrafast garnet growth must have occurred over a very limited pressure-temperature window. This example provides a rare glimpse into the discontinuous nature of mineral reactions in metamorphic rocks and highlights garnet as a unique recorder of the processes that occur when such rocks push toward equilibrium.