Variable oceanic lithosphere-forming processes in the vicinity of the Doldrums mega-transform zone, Mid Atlantic Ridge

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The proposed project will characterize the geochemical and isotopic variability of crustal and mantle rocks along a transect of the Mid Atlantic Ridge (MAR) from ~5-8°N. The aim is to better understand the mechanisms of oceanic lithosphere formation. On a regional scale (100s km) ridge magmatism may be influenced by plume-ridge interaction (100s km), but also varies on a more local scale (10-1 km) in response to the complex geodynamic conditions near or within oceanic (mega-) transform zones. On a regional scale, we will determine the along-axis frequency of geochemical-isotopic anomalies in Mid-Ocean Ridge Basalts (MORB) of the MAR from ~5-8°N. The new isotope data will constrain if ridge magmatism is influenced by nearby hotspots –which may have formed the nearby Sierra Leone Rise and Bathymetrist seamount chain– or if melting of laterally heterogeneous mantle is responsible for the geochemical and isotopic variability of MORB in the area. On a more local scale, changes in MORB chemistry approaching the Doldrums Mega-Transform Zone (MTZ) at ~7-8°N will allow evaluating how mantle melting and magma evolution processes respond to cooling due to the ‘cold-edge effect’ near ‘Ridge Transform Intersections’ (RTI). The complex geodynamic processes within the transform domain generated four ‘Intra-Transform Ridge’ (ITR) segments within the Doldrums MTZ. New bathymetric and geophysical data suggest that the peripheral ITR form in tectonically active areas characterized by low magma production rates. In contrast, more intense mantle melting and magmatic accretion characterize the two median ITR, despite being formed over a local mantle thermal minimum. The first-ever isotopic data of MORB from all four ITR segments will show if this apparent conflict can be reconciled by melting of anomalously fertile mantle in the median part of the Doldrums MTZ. Moreover, the rare opportunity to compare the geochemical-isotopic composition of mantle rocks (peridotites) and associated basalts from the western RTI will provide a complementary perspective on mantle composition and melt generation at the adjacent ITR, which probably formed after a recent eastward jump of the MAR axis. Mantle that has melted under the ridge before the ridge axis relocation would have migrated eastwards since, and may melt a second time under the newly formed ITR. If so, the geochemical and isotopic characteristics of the exposed mantle rocks and the associated basalts should show evidence for previous melt depletion. Overall, therefore, our study will provide novel constraints on how ridge magmatism responds to the different geodynamic conditions and mantle heterogeneity between different ridge segments on a regional scale, and near or within oceanic (mega-) transform zones on a local scale.