We use coupled zoned geochronology and thermodynamic modeling of garnet to elucidate the nature and scale of metamorphic dehydration during Eocene subduction of a quartzofeldspathic lithology from Sifnos, Greece. Two large garnet porphyroblasts were microdrilled to sample concentric growth zones, and these were dated using Sm–Nd geochronology. To put results in a geodynamic context and reveal the causes and consequences of garnet growth, we constructed thermodynamic forward models for a series of prescribed pressure–temperature (P–T) paths. Our data reveal three distinct phases of garnet growth: initial growth at 53.4±2.6 Ma (∼ 0.8 GPa and∼ 300° C), followed by a period of very limited growth until a second phase, at 47.22±0.36 Ma, and then a major pulse of growth, responsible for the majority of the final garnet volume, at 44.96±0.53 Ma (2.06–2.19 GPa and 490–550° C). This suggests a> 2 order of magnitude acceleration in volumetric growth rate from crystal core to rim, with the final growth pulse occurring rapidly (< 0.8 My), during a period of nearly isobaric heating at> 75° C/My. This final pulse was accompanied by net bulk rock dehydration of∼ 0.5 wt.%. Rapid heating during early stages of exhumation in the subduction channel, or by sharp thermal gradients related to slab-mantle coupling could be causes for this pulsed metamorphism and dehydration. The garnet data thus record a concentrated pulse of dehydration and heating during the otherwise slow and continuous process of subduction.