Hydrothermal Flow and Serpentinization in Oceanic Core Complexes Controlled by Mafic Intrusions

Deep-sea hydrothermal systems at slow/ultraslow-spreading mid-ocean ridges are often located within ultramafic rocks that are part of oceanic core complexes. These complexes contain lower-crustal and mantle sections exhumed due to detachment faulting. Hydrothermal circulation in these environments leads to massive sulfide deposits, hydration of oceanic lithosphere and conditions resembling early Earth's life origin. However, the relationship between hydrothermal pathways in these environments and crustal and mantle lithologies, faulting, magmatism, serpentinization and alteration is poorly understood. Here we present seismic models of a Mid-Atlantic Ridge core complex and its ultramafic-hosted hydrothermal system derived from full waveform inversion of controlled-source seismic data and from local earthquake tomography. The models and derived rock properties reveal high-permeability channels within serpentinized peridotite along the flanks of the core complex. These channels converge beneath active and fossil hydrothermal fields and are diverted around mechanically strong, impermeable shallow mafic intrusions (2-3 km wide, similar to 1 km thick), causing hydrothermal outflow and the formation of massive sulfide deposits around the intrusions' edges. These mafic intrusions also act as lids that limit fluid downflow-and thus serpentinization-in the centre of the core complex. Our results demonstrate that hydrothermal flow in ultramafic settings is controlled by lithology contacts, with mafic intrusions modulating hydrothermal pathways and extent of mantle serpentinization at depth.