Reaction‐Induced Mantle Weakening at High‐Pressure Conditions: An Example From Garnet Pyroxenites of Ulten Zone (Eastern Alps, N Italy)

Peridotites of Ulten Zone (Eastern Alps, N Italy) show a transition from coarse protogranular spinel lherzolites to fine-grained amphibole + garnet peridotites, recorded by the crystallization of garnet coronas around spinel. Pyroxenite veins, transposed along the peridotite foliation, show a similar metamorphic evolution from coarse-grained (garnet-free) websterites to fine-grained garnet websterites. In both peridotites and websterites, garnet previously exsolved from porphyroclastic high-temperature pyroxenes and later crystallized along the foliation. This evolution has been interpreted to reflect cooling and pressure increase of websterites and host peridotites from spinel- to garnet-facies conditions. Microstructures and crystallographic orientation data indicate that the re-equilibration of garnet websterites in the garnet stability field occurred during deformation. Porphyroclastic pyroxenes have been interpreted to deform by dislocation glide and creep. In particular, TEM observations indicate the activation of the (100)[010] slip system in orthopyroxene. Core-and-mantle microstructures also suggest that dislocation creep was aided by subgrain rotation recrystallization, leading to the formation of neoblastic pyroxenes. These recrystallized grains deformed by diffusion-accommodated grain boundary sliding, as indicated by the occurrence of quadruple junctions and straight, aligned grain boundaries. The transition from dislocation creep to diffusion creep in websterites was accompanied by the crystallization of garnet along foliation, which triggered the pinning of the recrystallized matrix and stabilized the fine-grained microtexture for diffusion creep, promoting rheological weakening. Garnet websterites of Ulten Zone thus offer a unique opportunity to investigate the effects of reaction softening during the corner flow in the supra-subduction lithospheric mantle induced by the descending slab.