[S-CG50] Rheology, fracture and friction in Earth and planetary sciences

[SCG50-P02] Numerous quadruple junctions of plagioclase detected in a gabbro ultramylonite across the Moho Transition Zone in Oman ophiolite

*Natsume Itsuki¹, Katsuyoshi Michibayashi¹ (1.Graduate School of Environmental Studies, Nagoya University)

Keywords:Quadruple junction, Plagioclase, Gabbroic ultramylonite, Superplasticity, Moho Transition Zone, Oman ophiolite

Where polycrystalline materials with grain sizes of a few µm are deformed at high temperatures, they can elongate several hundred percent or more without fracturing. This phenomenon is known as superplasticity. In superplasticity, the dominant deformation mechanism is thought to be grain boundary sliding, in which grains slide along grain boundaries and deform the entire specimen, so that its microstructure is characterized by very weak crystallographic preferred orientation with grain size of a few µm. (Edington et al., 1976). The occurrence of superplasticity in nature is proposed by some ultramylonites (e.g., Michibayashi and Oohara, 2013). There are some experimental studies on superplasticity using mantle materials (Hiraga et al., 2010). However, estimating whether superplasticity has occurred must be made with caution because fine grains do not necessarily mean that grain boundary sliding is dominant, and CPO weaken in transition areas of CPO type (e.g., Michibayashi and Oohara, 2013). Ashby and Verrall (1973) proposed that quadruple junctions, where four grains intersect at a single point on the plane, are formed by grain position exchange caused by grain boundary sliding. Quadruple junctions have been found from ultramylonite (Casini et al., 2021; Precigout et al., 2017; Menegon et al., 2013), and are treated as strong evidence for occurrence of grain boundary sliding. However, few study has documented their microstructural features.
We found numerous quadruple junctions of plagioclase in a gabbroic ultramylonite occurred in the Moho Transition Zone in Oman ophiolite. In this study, the deformation history of the ultramylonite will be discussed based on the distribution of quadruple junctions, in addition to the analysis of grain size and crystallographic preferred orientations.

References
Ashby and Verrall, 1973, Acta Metall, 21, 149-163.
Casini et al., 2021, J. Geophys. Res., 126, e2021JB022215.
Edington et al., 1976, Prog. Mater. Sci., 21, 61-170
Hiraga et al., 2010, Nature, 468, 1091-1094.
Precigout et al., 2017, Nat. Commun., 8, 15736.
Menegon et al., 2013, J. Struct. Geol., 48, 95-112
Michibayashi and Oohara, 2013, Earth Planet. Sci. Lett., 377, 299-310.