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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 drop 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. Key Points: MAR 13°20′N corrugated detachment fault is composed of pervasively silicified mafic breccias overplated from hangingwall diabasesQuartz fluid inclusions record mixing of hangingwall silica‐rich brines with footwall serpentinite‐derived fluids (H2 ± CH4) in detachmentThis heterogeneous fluid circulation in shallow detachment fault is inconsistent with models of detachments channeling deep fluid flow [ABSTRACT FROM AUTHOR]

Fluid inclusions trapped in quartz veins hosted by a leucogneiss from the southern part of the Naxos Metamorphic Core Complex (Attic-Cycladic-Massif, Greece) were studied to determine the evolution of the fluid record of metamorphic rocks during their exhumation across the ductile/brittle transition. Three sets of quartz veins (V-M2, V-BD & V-B) are distinguished. The V-M2 and V-BD are totally or, respectively, partially transposed into the foliation of the leucogneiss. They formed by hydrofracturing alternating with ductile deformation accommodated by crystal-plastic deformation. The V-B is discordant to the foliation and formed by fracturing during exhumation without subsequent ductile transposition. Fluids trapped during crystal-plastic deformation comprise two very distinct fluid types, namely a CO2-rich fluid and a high-salinity brine, that are interpreted to represent immiscible fluids generated from metamorphic reactions and the crystallization of magmas respectively. They were initially trapped at ∼625 °C and 400 MPa and then remobilized during subsequent ductile deformation resulting in various degrees of mixing of the two end-members with later trapping conditions of ∼350 °C and 140 MPa. In contrast, brittle microcracks contain aqueous fluids trapped at 250 °C and 80 MPa. All veins display a similar δ13C pointing to carbon that was trapped at depth and then preserved in the fluid inclusions throughout the exhumation history. In contrast, the δD signature is marked by a drastic difference between (i) V-M2 and V-BD veins that are dominated by carbonic, aqueous-carbonic and high-salinity fluids of metamorphic and magmatic origin characterized by δD between −56‰ and −66‰, and (ii) V-B veins that are dominated by aqueous fluids of meteoric origin characterized by δD between −40‰ and −46‰. The retrograde P- T pathway implies that the brittle/ductile transition separates two structurally, chemically and thermally distinct fluid reservoirs, namely (i) the ductile crust into which fluids originating from crystallizing magmas and fluids in equilibrium with metamorphic rocks circulate through a geothermal gradient of 30 °C km−1 at lithostatic pressure, and (ii) the brittle upper crust through which meteoric fluids percolate through a high geothermal gradient of 55 °C km−1 at hydrostatic pressure. [ABSTRACT FROM AUTHOR]