Mineral structural formula re-calculations (atoms per formula unit, apfu) based on 28 oxygen equivalents and derived late-stage hydrothermal crystallization temperatures were obtained from vein-filling chlorite cores within the proximal metasomatic zones of the Onnuri Oceanic Core Complex, Central Indian Ridge, during the 2019 and 2022 R/V ISABU dredge operations (RD1912, RD1913, RD2208). The aim was to place precise thermodynamic constraints on the late-stage cooling profile of focused V1 hydrothermal conduits, defining the thermal baseline prior to crystallization-driven fracturing. EPMA major element datasets of vein-center chlorites were processed through structural inversion frameworks. Crystallization temperatures were computed and cross-validated using two independent geothermometers: the empirical AlIV-in-chlorite model of Cathelineau (1988) and the semi-empirical low-to-medium temperature thermodynamic approach of Bourdelle et al. (2013).