Heterogeneity in texture and crystal fabric of intensely hydrated ultramylonitic peridotites along a transform fault, Southwest Indian Ridge

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Kakihata, Y., Michibayashi, K., & Dick, H. Heterogeneity in texture and crystal fabric of intensely hydrated ultramylonitic peridotites along a transform fault, Southwest Indian Ridge. Tectonophysics, 823, (2022): 229206, https://doi.org/10.1016/j.tecto.2021.229206.
Microstructures and olivine crystal fabrics were studied in amphibole-bearing peridotite samples obtained from the Marion Fracture Zone of the Southwest Indian Ridge by dredge D19 of the 1984 PROTEA Expedition Leg 5 cruise of the RV Melville. The peridotites show various textures ranging from extremely fine-grained well-layered ultramylonites to heterogeneously strained tectonites. Electron back-scatter diffraction analyses revealed that olivine crystal-preferred orientations (CPOs), which are developed primarily in coarse granular peridotites in the mantle, become weaker with an increasing degree of grain-size reduction from coarser to finer grains, for both porphyroclastic and matrix olivine grains. However, two well-layered ultramylonites are characterized by bimodal CPOs of (010)[001] (B type) and (001)[100] (E type) or a strong maximum of [010] normal to the foliation and girdle patterns of both [100] and [001] on the foliation plane (i.e., an axial [010] pattern or AG type). Moreover, spinel grains within these well-layered ultramylonites have not only been broken down to form olivine and amphibole by hydrous reactions, but have also been fractured and their fragments pulled apart in the fine-grained matrix. These features indicate that shear deformation occurred as increasing stress under hydrous conditions during the final stage of deformation, which enabled the local occurrence of low-temperature plastic deformation, resulting in the development of a CPO and a foliation within the ultramylonites.
This study was supported by research grants awarded to K.M. by the Japan Society for the Promotion of Science (Kiban-A 22244062, Kiban-S 16H06347). H.J.B.D. was supported by the US National Science Foundation (NSF/MG&G) and Woods Hole Oceanographic Institution.