Linking Reactive Fluid Flow to Rheology of Eclogite-Facies Oceanic Crust

During subduction, progressive heating and burial drive dehydration reactions that release H₂O-rich fluids from altered oceanic crust. Under sub-arc conditions (2–4 GPa), the transition from blueschist- to eclogite-facies mineral assemblages is accompanied by the release of substantial amounts of water (up to ~5 wt.%). If fluid transport occurs on timescales that are short relative to subduction, these fluids migrate through the overlying oceanic crust into the mantle wedge or along the slab–wedge interface. This process is critical for the generation of arc magmas and for their enrichment in fluid-mobile trace elements. Despite the importance of this process, relatively few direct constraints exist on the extent to which such fluids react with the eclogite-facies crust through which they migrate.

Here, we present field, petrological and geochemical observations from a pristine suite of transport veins preserved in mafic eclogites of the Tauern Window, Eastern Alps. The veins are dominated by high-variance, quartz-rich mineral assemblages and are surrounded by well-developed, omphacite-dominated selvages. Phase equilibrium modeling indicates that vein formation occurred at or near peak pressure–temperature conditions of ~2.5 GPa and ~600 °C. A striking feature of the fluid–rock interaction is the near-complete consumption of garnet by the reactive fluid. Trace-element zoning in partially reacted garnet porphyroblasts records a fluid-driven dissolution–precipitation mechanism that mobilized middle and heavy rare earth elements (MREE and HREE). Isocon analysis of the altered eclogite selvages reveals bulk gains in Na and Li, accompanied by losses of REE, Sr, K, Cu, Fe, Al, Y, Mn, Ba, and Cr, while Ni, Sc, and Ti appear to have been conserved.

Pure omphacite layers and seams are commonplace throughout the Eclogite Zone and are interpreted as sealed transport veins. The associated microstructures record embrittlement and fracturing following fluid–rock interaction. Collectively, these observations indicate that reactive fluid flow under eclogite-facies conditions may influence the rheology of subducting oceanic crust.