The MAR 13°20’N corrugated detachment fault is composed of pervasively silicified mafic cataclastic breccias, instead of ultramafics and gabbros commonly found at other detachments. These breccias record overplating of hangingwall diabases, with syntectonic silicification due to important influx of silica‐iron‐rich fluids, able to leach alkalis and calcium. Fluids trapped in quartz inclusions show important salinity variations (2.1‐10 wt.% NaCl eq.) indicating supercritical phase separation. Fluid inclusions also contain minor amounts of H2±CO2±CH4±H2S, with high H2/CO2 and H2/H2S ratios, signatures typical of ultramafic‐hosted vent fluids. We propose that seawater infiltrated the hangingwall upper crust at the axis adjacent to the active detachment, reaching a reaction zone at the dyke complex base (∼2 km). At >500°C, fluids become Si‐rich during diabase alteration (amphibolite‐facies alteration in clasts), and undergo phase‐separation. Brines, preferentially released in the nearby detachment fault during diabase brecciation, mix with serpentinite‐derived fluids bearing H2 and CH4. Cooling during detachment deformation and fluid upward migration triggers silica precipitation at greenschist‐facies conditions (quartz+Fe‐rich‐chlorite±pyrite). Important variations in fluid inclusion salinity and gas composition at both sample and grain scales record heterogeneous fluid circulation at small spatial and short temporal scales. This heterogeneous fluid circulation operating at <2 km depth, extending both along‐axis and over time, is inconsistent with models of fluids channeled along detachments from heat sources at the base of the crust at the fault root. Present‐day venting at detachment footwall, including Irinovskoe, is instead likely underlain by fluid circulation within the footwall, with outflow crossing the inactive detachment fault near‐surface.

Plain language summary

Here we present constraints on fluid circulation along the 13°20′N oceanic detachment fault along the Mid‐Atlantic Ridge. Rocks recovered in situ with a deep‐sea robot yield mafic breccias, instead of serpentinized mantle rocks commonly found at other detachments. They likely originate from the base of the hangingwall dyke complex, brecciated during fault exhumation. These rocks are intensely silicified (quartz mineralization), resulting from upflow circulation of silica‐rich fluids derived from reactions with mafic rocks in a reaction zone. Fluid inclusion (micrometric cavities in quartz crystals that trapped circulating fluid) analyses reveal highly‐saline fluids likely formed by phase separation, while traces of hydrogen and methane likely record serpentinization. We thus propose that seawater infiltrated the crust down to a reaction zone at its base (2 km depth), where it became silica‐rich by rock hydrothermal alteration. Upon brecciation, these silica‐rich brines were released in the detachment where they mixed with fluids coming from footwall rock alteration. Temperature and pressure drops during fluid upflow promoted intense quartz crystallization. The active Irinovskoe hydrothermal site, sitting on the detachment fault ∼5 km off‐axis, is unrelated to fluid circulation in the detachment plane, and likely linked to a heat source within the footwall and directly below it.