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, Eastern Alps, preserve vein networks associated with eclogite-facies mineral assemblages. The structural diversity of these veins is encapsulated by two endmembers: Type I tensile veins that exhibit sharp contacts, discordant to the eclogite-facies foliation and Type II fluid segregates with non-planar morphologies. Compositional analysis reveals two main types of mafic eclogite: banded, low-silica and metagabbroic, high-silica compositions. Garnet growth occurred along a prograde P-T path between 2.0 ± 0.15 GPa, 585 ± 15°C and 2.5 ± 0.2 GPa, 630 ± 10°C, consistent with conditions on the slab-wedge interface of modern subduction zones. Dehydration of lawsonite and Na-amphibole released ~5 wt.% H₂O over 20-30°C, creating ~13% 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 ~80°C, suggesting that pockets of undrained fluid persisted in the oceanic crust for 105-106 years during subduction to sub-arc depths. A simple petrological-mechanical model for the blueschist-to-eclogite transition shows that under extremely low permeabilities (10-22 to 10-34 m2), Type I veins may form by tensile failure during periods of high pore fluid pressure, whereas Type II fluid segregates represent accumulations of locally derived fluids during periods of lower fluid pressure. The rate of lawsonite breakdown is intimately linked to hydraulic fracturing and embrittlement of the oceanic crust at depths where intermediate-to-deep seismicity occurs in cold subduction zones.