Geochemistry of lava samples collected near the oceanic detachments at 13°N along the Mid-Atlantic Ridge
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The structure and composition of the oceanic lithosphere is mainly controled by the supply of magmatic melts to the ridge axis. At slow-spreading, mid-oceanic ridges are characterized by complex spreading styles, where large variations in the melt supply are resulting in intermittent volcanism. During periods of reduced magmatic activity, spreading is accomodated by displacement on low-angle extensional detachement faults, forming Oceanic Core Complexes (OCC). These deep-seated faults are ultimately exhuming lower crust and upper mantle material, exposing all types of eruptive and plutonic igneous rocks, as well as mantle-derived ultramafic rocks variably altered and deformed (MacLeod, J. Escartín et al. 2002, Escartín, Mével et al. 2003, Dick, Tivey et al. 2008). This composite geological setting is suggesting a complex history and interplay between tectonic deformation and magmatic supply around OCCs.The aim of this study is to characterize the spatiotemporal evolution of magmatic processes concomitant with the development of a detachment fault. The foundation of this geochemical work is a recent detailed study of the tectonic structure and evolution of detachment fault zones at 13°20’N and 13°30’N along the Mid-Atlantic Ridge (Escartín, Mével et al. 2017). Highresolution microbathymetry, coupled with samples collected by deepsea vehicles are allowing us to present a geochemical dataset integrated in a complex OCC geological landscape. New major, trace elements and Sr, Nd, Pb and Hf isotopes, together with previously published data (Wilson, Murton et al. 2013), are used to build a petrogenetic model. Geochemical variance comparable to that of the entire Mid-Atlantic Ridge suggest a heterogenous mantle source as well as complex melting- and postmelting modifications. Enriched melt signals is best explaned by includiong a fusible, recycled mantle component, whereas depleted signals require the presence of a anomalous residual mantle component. Off-axis volcanism through permeable fault zones is indicated by the presence of insitu, unevolved basalts in fault disrupted areas. Prolonged periods of melt starvation and enhanced crustal accretion by fault displacement is likely to reflect the combination of two different mechanisms: (1) reduced influen the recycled, fusible mantle component, and (2) diversionof melts away from the spreading axis fault zone conduits.
PublisherThe University of Bergen
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