[S-CG46] New Insights of Fluid-Rock Interactions: From Surface to Deep Subduction Zone

[SCG46-13] Analysis of water-rock reactions in fault rocks using the pseudosection: an example from a ductile shear zone in the crust-mantle boundary of the Oman ophiolite

*Natsume Itsuki^1,2, Yui Kouketsu¹, Katsuyoshi Michibayashi^1,3, Atsushi Okamoto⁴ (1.Department of Earth and Planetary Sciences, GSES, Nagoya University, 2.Kanagawa Prefectural Museum of Natural History, 3.Volcanoes and Earth’s Interior Research Center, IMG, JAMSTEC, 4.Research School of Environmental Studies, Tohoku University)

Keywords:pseudosection, water-rock reactions, fault rocks, the Oman ophiolite, the crust-mantle boundary, ductile shear zone

A ductile shear zone developed at the crust-mantle boundary in the Oman ophiolite has been studied in terms of the microstructures and crystal-fabrics within the fault rocks (Michibayashi and Oohara, 2013). In this study, we applied the pseudosection method to understand the water-rock reactions that produce hydrous minerals during deformation at the crust-mantle boundary of the oceanic plate. The protolith is a Moho transition zone composed of massive gabbro with dunitic fragments. Based on microstructural observations, the fault rocks were classified into five textures: Ol texture consisting of olivine, Amp-Pl texture consisting of amphibole and plagioclase, Amp texture consisting of amphibole, chlorite, and serpentine, Chl texture consisting of chlorite, and Cpx texture consisting of clinopyroxene and chlorite. Subsequently, a pseudosection between the bulk composition and temperature was drawn under the assumption of a Ca-Fe-Mg-Al-Si-O-H system saturated with H2O fluid and a pressure of 200 MPa corresponding to the thickness of a typical oceanic crust. In this study, we set up the bulk composition as a function of the mixing ratio between dunite and gabbro. As a consequence, the five textures within the fault rocks can be explained by pseudosection as follows. The Ol and Amp-Pl textures were mostly derived from dunite and gabbro, respectively, and both textures were formed by a water-rock reaction at approximately 750°C. The Amp texture occurred at temperatures below ca. 750°C, whereas the Chl and Cpx textures occurred below ca. 650°C, regardless of the mixing ratio between the dunite and gabbro. Moreover, our pseudosection revealed how serpentine minerals and talc could occur at lower temperatures.

Reference: Michibayashi & Oohara, 2013, Earth Planet. Sci. Lett., 377, 299-310.