U-Th-Ra Isotope Evidence for Rates of Fluid Transport and Magmatic Processes Beneath Santorini, Aegean Volcanic Arc, Greece

Abstract

The island of Santorini is situated in the Aegean and is part of the Hellenic Volcanic Arc, formed by the northward subduction of the African Plate under the Eurasian Plate. Santorini has been volcanically active since the middle Pleistocene. The two youngest (<200 ka) volcanic sequences are the second explosive cycle that terminated with the caldera forming Minoan eruption of ~1600 BC, and the Kameni island dacites, which comprise twelve historic lava eruptions from 46 AD to 1950. Selected rocks from these two sequences have been analysed for major and trace elements, for Sr and Nd isotopes, and for the short lived isotopes of U-Th-Ra by TIMS. Second eruptive cycle The compositions of the second eruptive cycle range from basaltic andesite to rhyolite, and samples are enriched in LREE, LILE, U and Th. Sr, Nd and Th isotopic compositions are also enriched relative to MORB, and they preserve good isotope-isotope correlations with S7Sr/S6Sr = 0.7035-0.7052 and 143Nd/144Nd = 0.51285-0.51267 (Fig. 1), and (23~ = 0.97-0.90. The rocks tend to have similar (23~ and (238U/232Th) activity ratios, and they are therefore in 23~ equilibrium. The within-suite trace element and Sr-Nd isotope trends can be modelled by AFC processes, indicating that variable assimilation of up to ~3% upper crustal material was responsible for the range in Sr, Nd and Th isotopes in the second eruptive cycle. The composition of the inferred pre-contamination magma reflects contributions from both sedimentderived melts (-0.3%) and hydrous fluids from the subducted slab. Relative to N-MORB, the enrichment in LREE, the high Th/Ta ratios and the low initial 143Nd/144Nd [~ 0.51285] and Th activity ratios [(23~ ~ 0.6] reflect modification of the mantle wedge by small degree melts of the subducted sediment. In contrast, the concentrations of fluid mobile trace elements [Rb, Sr, Ba, K, U and Pb] are significantly increased by introduction of slabderived fluids. The data are consistent with a fluid Sr isotope ratio of ~0.7035, as inferred from studies of other island arcs (e, g. Turner et al., 1997), and mass balance considerations indicate that fluid addition accounts for ~40% of the U budget. The