TY - JOUR A1 - Linckens, Jolien A1 - Tholen, Sören Matthias T1 - Formation of ultramylonites in an upper mantle shear zone, Erro-Tobbio, Italy T2 - Minerals N2 - 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. KW - ultramylonite KW - shear zone KW - phase mixing KW - EBSD KW - Erro-Tobbio peridotite KW - melt-rock reaction Y1 - 2021 UR - http://publikationen.ub.uni-frankfurt.de/frontdoor/index/index/docId/63392 UR - https://nbn-resolving.org/urn:nbn:de:hebis:30:3-633926 SN - 2075-163X N1 - This research was funded by the Deutsche Forschungsgemeinschaft, grant number LI 2888/2-1. VL - 11 IS - 10, art. 1036 SP - 1 EP - 22 PB - MDPI CY - Basel ER -