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 the formation of neoblastic pyroxenes. These recrystallized grains deformed by diffusion-accommodated grain boundary sliding, as indicated by the occurrence of four grain 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 the 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.

Reaction-induced mantle weakening at high pressure conditions: an example from garnet pyroxenites of Ulten zone (Eastern Alps, N Italy) / L. Pellegrino, L. Menegon, S. Zanchetta, F. Langenhorst, K. Pollok, S. Tumiati, N. Malaspina. - In: JOURNAL OF GEOPHYSICAL RESEARCH. SOLID EARTH. - ISSN 2169-9313. - (2021). [Epub ahead of print] [10.1029/2021JB022584]

Reaction-induced mantle weakening at high pressure conditions: an example from garnet pyroxenites of Ulten zone (Eastern Alps, N Italy)

S. Tumiati
Penultimo
;
2021

Abstract

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 the formation of neoblastic pyroxenes. These recrystallized grains deformed by diffusion-accommodated grain boundary sliding, as indicated by the occurrence of four grain 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 the 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.
Settore GEO/07 - Petrologia e Petrografia
Settore GEO/06 - Mineralogia
Settore GEO/02 - Geologia Stratigrafica e Sedimentologica
PRIN201719SPOLI_01 - The Dynamic Mass Transfer from Slabs to Arcs (Dynastar) - POLI, STEFANO - PRIN2017 - PRIN bando 2017 - 2019
24-nov-2021
https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021JB022584
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2434/885895
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