A key role for diffusion creep in eclogites: Omphacite deformation in the Zermatt-Saas unit, Italian Alps

Abstract

Eclogites commonly form from subducted oceanic crust and as such carry key information on subduction zone rheology. Using a combination of microanalytical techniques on deformed eclogites from the Zermatt-Saas unit of the Italian Alps, we explore the mechanisms which resulted in both an omphacite shape and lattice preferred orientation. Omphacite defines both foliation and lineation in these rocks and displays a strong S-type lattice preferred orientation. Scarcity of microstructures associated with dislocation creep combined with observed sharp asymmetrical chemical zonation in foliated grains suggest diffusion creep plays an important role in the development of omphacite lattice preferred orientation in these eclogites. Modelling of the pressure-temperature conditions possible for observed mineralogy and mineral geochemistry, and textural relationships between omphacite and retrogressive minerals, place the action of diffusion creep, at the latest, by the onset of retrogression of these Zermatt-Saas eclogites. We propose a model of eclogite deformation that occurred initially via small amounts of dislocation creep which moved quickly into a dominant diffusion creep field, particularly as exhumation/retrogression of these eclogite rocks began. This result suggests that diffusion creep can dominate eclogite deformation at high P-T conditions in subduction zones.