High-spatial-resolution major element chemical compositions (wt.%) and computed structural XFeO ratios [FeO/(FeO+MgO)] of coexisting silicate phases were measured on thin-section textural domains of samples recovered from the Onnuri Oceanic Core Complex detachment fault zone, Central Indian Ridge, during the 2019 and 2022 R/V ISABU dredge operations (RD1912, RD1913, RD2208). The aim was to track mineral-scale chemical evolution, local element partitioning, and micro-textural iron sequestration across the matrix and distinct hydrothermal vein networks (V1, V2, V3). In-situ chemical mapping and spot analyses were conducted using a JEOL JXA-8530F Field-Emission Electron Probe Micro-Analyzer (FE-EPMA) at Seoul National University. Operating conditions were set to an accelerating voltage of 15 kV and a beam current of 10 nA. A defocused beam (5 μm diameter) was strictly utilized to prevent beam-induced volatilization of structural hydroxyl groups in volatile-sensitive hydrous silicates (serpentine, talc, tremolite, chlorite, anthophyllite).