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