Pulsed dehydration and garnet growth during subduction revealed by zoned garnet geochronology and thermodynamic modeling, Sifnos, Greece

- Garnet geochronology and thermodynamic modeling reveal nature of Aegean subduction.
- Garnet growth is used as a proxy for metamorphic dehydration.
- Pulsed metamorphism and rapid dehydration during subduction.

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.