Fluid-Rock Interaction at Sub-Arc Conditions: A Petrological Investigation of an Eclogite-Facies Vein System, Eclogite Zone, Eastern Alps

Abstract:

Subduction zones are the primary site of chemical differentiation on Earth. During prograde burial and heating, the breakdown of hydrous minerals releases fluid into the subducting slab; models for the genesis of arc magmas require fluid transport over long distances (>10 km) and short timescales (<1 Ma) into the mantle wedge, where it induces partial melting [1]. The release and movement of such fluids are also thought to play a pivotal role in generating and modulating subduction zone seismicity [2]. Direct constraints on the mechanics of fluid transport within the slab are thus of high value. In this study, we report petrographical and microtextural observations, and thermobarometric calculations from a brittle, eclogite-facies vein system in the Eclogite Zone of the Tauern Window, Eastern Alps. Host eclogites are fractured by 10-50 cm diameter quartz veins containing syntaxial, idiomorphic omphacite, epidote and kyanite. Omphacite-rich reaction selvages mantle vein margins and transition toward pristine eclogite over distances of <30 cm. Polyphase porphyroblastic garnet is present throughout the vein selvage and preserves a rich history of growth, dissolution and brittle deformation. Phase 1 garnet is characterized by typical Mn-enrichment in core domains and pseudosection calculations imply prograde growth at <20 kbar; phase 2 garnet is marked by a decrease in Fe²⁺, Ca, and Mn and an increase in Mg; phase 3 (rim) garnet is further enriched in Mg. Pseudosection calculations indicate peak metamorphic conditions of ~25 kbar, ~650 °C. Phase 1 and 2 garnet are cut by healed fractures, with compositions similar to phase 3 garnet; fracturing occurred at ~22 kbar, ~645 °C. These results suggest that deformation and cracking of the garnet and veining of the host rock occurred concurrently near peak metamorphic conditions, possibly as the result of fluid ingress.

[1] John, T., Gussone, N., Podlachikov, Y.Y., Bebout, G.E., Dohmen, R., Halama, R., Reiner, K., Magna, T., Seitz, H.-M., 2012. Volcanic arcs fed by rapid pulsed fluid flow through subducting slabs. Nat. Geosc. Lett. 5, 489-492.

[2] Hacker, B.R., Peacock, S.M., Abers, G.A. and Holloway, S.D., 2003. Subduction factory 2. Are intermediate‐depth earthquakes in subducting slabs linked to metamorphic dehydration reactions?. J. Geophys. Rsch.: Solid Earth, 108(B1).