Fluid flow at active oceanic core complexes, 13°N Mid-Atlantic Ridge

Oceanic core complexes (OCCs) are the result of long-lived, large displacement, low-angle detachment faults that expose lower crust and mantle rocks at slow-spreading mid-ocean ridges (MOR's). While OCCs share many structural and lithological features indicating some common tectonic processes of formation, until now there has been little constraint on whether fluid flow is related to their activity. Here, we describe recently acquired water column data and rock samples from several active OCCs (from near 13°N on the Mid-Atlantic Ridge) that reveal a history of high and low-temperature fluid flow. At the toe of the active OCC's, where the footwall emerges from beneath an uplifted wedge of fissured volcanics that forms the trailing edge of the hanging wall, massive sulphide chimneys and large volumes of mineralised talc mud indicate the passage of high-temperature hydrothermal fluids. The sulphides are a mixture of iron and copper sulphide in association with chalcedony and talc. The host rock is usually serpentinised peridotite mantle material although some greenschist diabase is also present in the form of dykes. Above the highest point of one of the active OCC's, CTD data revealed a plume of low salinity water. The plume was ~200m wide, had normal background temperature and was devoid of any particulates. We speculate that the origin of the high-temperature mineralization is hydrothermal circulation in the hanging wall, driven by intrusive volcanism injected from the subjacent neovolcanic accretion zones. The origin of the low-salinity plume is more elusive and could be a result of dehydration reactions of serpentinite to either fosterite or, with silicification, to talc. The latter mechanism would accord with the extensive outcrops of talc mud found near the OCC toe, but implies excessive volumes of rock in the reaction zone. Either way, the presence in unusual settings of fluid flow at OCC's indicates that fluid-rock reaction plays an important role at active OCC's.