| Summary: | The initial formation and temporal evolution of large igneous plateaus and the extent to which these large enigmatic igneous features impact on their immediate and distant ecological surrounding remains a matter of active research. The compositional variability in large igneous provinces has mainly been attributed to changes in the melting regime and shallow crustal processes and commonly ranges from depleted tholeiitic basaltic to enriched alkaline lavas. Large igneous provinces erupted in the submarine environment however, may also experience intense hydrothermal alteration during their formation resulting from an increased exchange between seawater and the erupting lavas during their eruptive history. The submarine Azores Plateau in the Central Northern Atlantic has generally been treated to represent such large igneous province formed since similar to 10 Ma by widespread volcanism and the unique tectonic regime which results in large fault systems exposing the erupted lavas. Here, we present new seismic, petrological and major element, trace element and isotope geochemical data from a similar to 1000 m stratigraphic section of submarine lavas exposed at the western Princessa Alice bank. The 22 samples recovered from the near-vertical rift wall provide evidence for intense water-rock exchange not observed anywhere in oceanic crust sampled to date. Fluid-immobile incompatible trace elements show that the samples formed from higher degrees of partial melting of a mantle source that is less enriched than the source that gives rise to the islands today. The extents of melting today are very small, implying a change in melting regime since initial formation of the Princessa Alice Plateau basalts that correspond to a melting anomaly in the Azores. Our observations indicate that the extreme levels of alteration may result from a combination of intensified magmatic activity during initial formation of the Azores Plateau and the tectonic regime providing pathways for the fluids. Our results impact on the ...
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