Depth-Dependent Chlorine Cycling at the Magmatic–Hydrothermal Interface of Mid-Ocean Ridges: Insights from Hornfelsic Diabase

Abstract The conductive boundary layer (CBL) at the magmatic–hydrothermal interface of mid-ocean ridges (MORs) regulates the fundamental exchange of heat and chemical species between cooling magmas and circulating seawater. While this interface is well-documented at shallow, fast-spreading ridges and in ophiolites, its nature in deep-seated magmatic environments—particularly along slow-spreading ridge segments hosting oceanic core complexes (OCCs)—remains poorly constrained. Here, we investigate the physicochemical properties of this interface through a detailed microstructural, geochemical, and fluid inclusion analysis of hornfelsic diabase, the primary constituent of the CBL, recovered from the Onnuri OCC on the Central Indian Ridge. Our results characterize two distinct lithologies: hornblende hornfelsic and pyroxene hornfelsic diabase. Clinopyroxene geothermometry yields peak metamor…