Mineralogy and geochemistry of lavas from the submarine lower caldera walls of Santorini Volcano (Greece)

Abstract

Santorini Volcano, located in the central sector of the South Aegean volcanic arc, is one of the most active and potentially dangerous volcanoes in Europe. It has hosted Plinian eruptions over the last 350 ka of which at least four eruptions were accompanied by caldera collapses. Even though Santorini Volcano is considered a major threat, the main focus of research, thus far, has been on the comparatively young and subaerial deposits, whereas older stages of volcanism have been poorly studied. Our study focusses on samples from the submarine inner caldera walls and gives new insights into the early evolutionary stages of Santorini Volcano, contributing to a better understanding of its eruptive history and thus potential risks. The submarine lava successions were sampled along the inner caldera wall by a remotely operated vehicle (ROV) during R/V POSEIDON cruise 511 in 2017.The recovered lavas comprise two magmatic series, a low-K basaltic series overlain by a medium- to high-K series, which includes basaltic andesites, andesites and occasional dacites to trachytes. Major and trace element compositions and mineral zonation patterns suggest that fractional crystallisation and periodic magma replenishment were the dominant processes controlling magma evolution of both magmatic series. In addition, repeated magma mixing events played an important role, as indicated by zonation patterns in plagioclase and clinopyroxene ante- and phenocrysts. The thickness of the submarine lava successions (≥100 m) and the occurrence of similarly zoned plagioclase throughout indicate long-lived magma plumbing systems characterised by repeated differentiation and magma mixing. Furthermore, the incompatible element and radiogenic isotope ratios indicate a heterogeneous mantle source for Santorini magmas, which reflects the relative contribution of subduction (sedimentary) input and mantle wedge influx. A lava sample from the northern submarine caldera wall probably represents a deep level of the original Peristeria volcano. 40Ar/39Ar dating of andesite lavas from the southern submarine caldera produced ages of ~255 ka, whereas one basalt lava produced an age of 309 ± 30 ka. Interestingly, the new ages of both groups fall within an age gap in the volcanic history of Santorini Volcano. Even though it was not possible to unequivocally correlate the sampled submarine lava series to known subaerial units, the major and trace element compositions, and Sr-Nd-Pb isotopic compositions of our intermediate lavas show a strong similarity to subaerial lavas of Peristeria volcano, the second oldest major stage of Santorini volcanism. It however seems more likely that we have sampled a thus far unknown stage of volcanism at Santorini Volcano.