Abstract
Deformation in the upper mantle is localized in shear zones. In order to localize strain, weakening has to occur, which can be achieved by a reduction in grain size. In order for grains to remain small and preserve shear zones, phases have to mix. Phase mixing leads to dragging or pinning of grain boundaries which slows down or halts grain growth. Multiple phase mixing processes have been suggested to be important during shear zone evolution. The importance of a phase mixing process depends on the geodynamic setting. This study presents detailed microstructural analysis of spinel bearing shear zones from the Erro-Tobbio peridotite (Italy) that formed during pre-alpine rifting. The first stage of deformation occurred under melt-free conditions, during which clinopyroxene and olivine porphyroclasts dynamically recrystallized. With ongoing extension, silica-undersaturated melt percolated through the shear zones and reacted with the clinopyroxene neoblasts, forming olivine–clinopyroxene layers. Furthermore, the melt reacted with orthopyroxene porphyroclasts, forming fine-grained polymineralic layers (ultramylonites) adjacent to the porphyroclasts. Strain rates in these layers are estimated to be about an order of magnitude faster than within the olivine-rich matrix. This study demonstrates the importance of melt-rock reactions for grain size reduction, phase mixing and strain localization in these shear zones.
Highlights
Strain in the upper mantle is inhomogeneously distributed
This study presents detailed microstructural analysis of spinel bearing shear zones from the Erro-Tobbio peridotite (Italy) that formed during pre-alpine rifting
This study demonstrates the importance of melt-rock reactions for grain size reduction, phase mixing and strain localization in these shear zones
Summary
Strain in the upper mantle is inhomogeneously distributed. The deformation is localized in zones of high strain, i.e., shear zones. The strength of the CPO will increase with increasing strain, when strain is accommodated by dislocation creep, leading to a positive feedback (e.g., [17,26]) This process has been suggested as being important for shear localization in an upper mantle shear zone with no large grain size variation [15] and for the reactivation of pre-existing shear zones [16]. A softening does occur when there is a change from dislocation creep to a grain size-dependent deformation mechanism It has been shown in deformation experiments on olivine that disGBS is an important deformation mechanism under upper mantle conditions [13]. Due to the extensive melt percolation during deformation, the spinel bearing shear zones of the Erro-Tobbio peridotite are ideal to analyze the importance of melt-rock reactions on phase mixing and strain localization. The strain rate in these layers is estimated to be one order of magnitude faster than in the olivine-rich matrix
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