Oceanic detachment faults focus very large volumes of black smoker fluids

Abstract

International audiencet is generally assumed that the seawater-derived fluids that feed black smoker vent fields on the seafloor are discharged vertically from depths of ∼1–3 km. We present new oxygen and strontium isotope data that show that fluids at black smoker temperatures of 300–400 °C were focused along a low-angle detachment fault at 15°45′N near the Mid-Atlantic Ridge. Isotopic alteration is the most extreme ever reported from oceanic rocks altered at similar temperatures, indicating intensely focused fluid flow both in recharge and discharge parts of the hydrothermal system. Rare earth element mobility in the fault rocks demonstrates isotopic alteration by evolved hydrothermal fluids, not conductively heated seawater. The fault zone protolith was predominantly ultramafic, but also included mafic rocks, with metasomatic alteration to talc-tremolite-chlorite schists resulting mainly from chemical exchange between these lithologies during fluid flow. Fluids in equilibrium with this assemblage would be similar to ultramafic-hosted black smoker fluids. We present a new model in which hydrothermal circulation around detachment faults evolves from basalt hosted (TAG type), to footwall ultramafic hosted (Rainbow type), to low-temperature ultramafic hosted (Lost City type). Key features of our model are the intrusion of gabbro bodies immediately below the detachment to provide a heat source for circulation, and focusing of fluid flow into the detachment fault to allow venting away from the neovolcanic axis

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“…Research on detachment faulting and OCCs indicates that this style of oceanic spreading is intimately linked to hydrothermal circulation and encompasses a wide variety of fluid flow and hydrothermal regimes (McCaig et al, 2007). High-temperature fluid circulation is well documented mineralogically and geochemically (Schroeder and John, 2004;Boschi et al, 2006;McCaig et al, 2010), and uplift along detachments may promote circulation and alteration within the footwall (Andreani et al, 2007).…”

“…High-temperature fluid circulation is well documented mineralogically and geochemically (Schroeder and John, 2004;Boschi et al, 2006;McCaig et al, 2010), and uplift along detachments may promote circulation and alteration within the footwall (Andreani et al, 2007). McCaig et al (2007) propose temporal evolution in the style of hydrothermal circulation associated with the development of OCCs: from high-temperature systems hosted in the basaltic hanging wall within the rift valley (e.g., Trans-Atlantic Geotraverse [TAG]-type), to high-temperature ultramafic-hosted systems within the footwall (Rainbowtype), and ultimately to off-axis ultramafic-hosted systems within the footwall (Lost City-type). At Atlantis Massif we can study the first and third of these circulation types.…”

“…Sampling at different locations along the former detachment fault will allow us to evaluate records of fluid-rock interaction and metasomatism and test the hypothesis that exhumation faults serve as long-lived permeable pathways for hydrothermal flow that can feed moderate-to high-temperature hydrothermal vents (McCaig et al, 2007). In ultramafic/mafic systems, assemblages of talc-tremolite-chlorite form at >350°C (Boschi et al, 2006(Boschi et al, , 2008McCaig et al, 2010), typical of black smoker discharge zones, whereas serpentine-prehnite-hydrogarnet assemblages form at lower temperatures (Frost et al, 2008;Bach and Klein, 2009).…”

“…An estimated 40% or more of the Atlantic Ocean floor was formed by interlinked processes of asymmetric extension, magmatism, and detachment faulting resulting in exposure of mantle and lower crustal rocks in OCCs (Dick et al, 2008;Cannat et al, 2006;Smith et al, 2006Smith et al, , 2008Ildefonse et al, 2007;Escartín et al, 2003;Tucholke et al, 2008). OCCs incorporate highly reactive olivine-rich rocks that interact with seawater over a range of temperatures to produce both high-and low-temperature hydrothermal systems (Douville et al, 1997(Douville et al, , 2002Charlou et al, 1998Charlou et al, , 2002Sagalevich et al, 2000;Kelley et al, 2005;McCaig et al, 2007). Chemical exchange between the mantle and the hydrosphere supports abundant microbial communities in regions of focused fluid discharge in both types of systems (Schrenk et al, 2004;Takai et al, 2004;Nercessian et al, 2005;Brazelton et al, 2006;Campbell et al, 2006;Perner et al, 2007).…”

Central role of detachment faults in accretion of slow-spreading oceanic lithosphere

“…There is also evidence for hightemperature fluid circulation along detachment faults 20,21 , suggesting a direct link between hydrothermal activity and deformation associated with the faults. The data thus point to a link, albeit complex, between extension on detachments and hydrothermal activity, consistent with a thicker lithosphere at asymmetrical relative to symmetrical ridge sections.…”