Constraints on the Dehydration Systematics of Subducted Oceanic Crust Across the Blueschist-to-Eclogite Facies Transition (Eclogite Zone, Eastern Alps)

Mafic eclogites of the Tauern Window in the Eastern Alps preserve vein networks associated with eclogite-facies mineral assemblages. The structural and mineralogical diversity of these veins is encapsulated by Type I veins, which resemble deformed tension gashes, and Type II quartz segregates with non-planar morphologies. Within host eclogites, garnet growth occurred along a prograde P-T path between 2.05 ± 0.10 GPa, 580 ± 15°C and 2.50 ± 0.10 GPa, 630 ± 15°C, consistent with conditions on the slab-wedge interface of modern subduction zones. The dehydration of lawsonite and Na-amphibole released ∼5 wt.% H₂O over 20–35°C, creating ∼11% transient porosity. In situ oxygen isotope analysis of quartz-rutile pairs constrains formation temperatures to between 460°C and 610°C for Type I and II vein structures. Individual veins preserve records of protracted crystallization over ∼100°C, suggesting that fluids remained undrained in the oceanic crust for 10⁵–10⁶ years during subduction to ∼90 km. A simple petrological-mechanical model for the blueschist-to-eclogite transition shows that under extremely low permeability (10⁻²² to 10⁻³⁴ m²), Type I veins may form by tensile failure during periods of high pore fluid pressure, whereas Type II quartz segregates represent accumulations of derived fluids during periods of lower fluid pressure. These findings imply that domains of oceanic crust with extreme low permeability may retain fluids released during the blueschist-to-eclogite past the depths of arc magma genesis.