The compositional variability in large igneous provinces has mainly been attributed to changes in the melting regime and shallow crus- tal 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 10 Ma by widespread volcanism and the unique tectonic regime which results in large fault systems exposing the erupted lavas. The dataset here consists of new seismic, petrological and major element, trace element and isotope geochemical data from a 1000 m stratigraphic section of submarine lavas exposed at the western Princessa Alice bank in the Azores archipelago. The 22 samples recovered during two dives of ROV Marum Quest during M128 Azores Plateau in 2016 from the near-vertical rift wall provide evidence for intense water-rock exchange not observed anywhere in oceanic crust sam-pled to date. The dataset consists of mineral major element, trace element and O-isotope data and whole rock major element, trace element and Sr-isotope data. 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 interpretation of shallow level crustal magmatic processes, in which the contribution of crustal sources to the ascending melt may be different to what had previously been suggested. We propose that hydrothermal alteration during submarine igneous plateau forming events can drastically change the compositions of the igneous crust. The associated elemental and isotopic exchange between the oceanic crust and hydrosphere may substantially change the chemical fluxes between oceans and crust during the emplacement of oceanic plateaus.