Petrogenetic Processes and their Timescales beneath Santorini, Aegean Volcanic Arc, Greece

Abstract

An understanding of the timescales of petrogenetic processes at destructive plate margins is critical to assess models of fluid and magma transport through the mantle wedge and magma storage within the overriding crust. The Aegean Volcanic Arc is formed by the northward subduction of the African Plate under the continental Aegean microplate. Shallow ( 65 km) teleseismic and microseismic activity in the subducting slab form linear trends from the transform segments in the trench towards the volcanic centres of Methana, Milos, Santorini and Nisyros. Epicentre trends are approximately collinear with the direction of subduction at the trench. This pattern implies fracturing of the slab at the trench due to the geometry of the plate margin, and subsequent slab deformation along zones of weakness. The coincidence of the seismicity with volcanic activity along the arc suggests that pore fluids are collected within fault zones and are released by hydraulic fracturing to trigger localised melting within the wedge. Santorini, in the central part of the arc, has been volcanically active since the middle Pleistocene. The two youngest volcanic sequences are the second eruptive cycle (SEC; ~172 ka-3.6 ka) and the Kameni island lavas (46 AD to 1950). For the SEC, major and trace element and Sr and Nd isotope data (87Sr/86Sr = 0.7035-0.7052, correlated with 143Nd/144Nd = 0.51285-0.51267) indicate that differentiation of the less evolved samples is dominated by fractional crystallisation from a primary magma with low H2O content ( 60 wt% SiO2. The Kameni dacites have restricted major, trace and Sr and Nd isotope variations (SiO2 = 65-68 wt%, 87Sr/86Sr = 0.70475, 143Nd/144Nd = 0.51274). They contain mafic enclaves with distinctly different incompatible element ratios. Concared to N-MORB, samples are enriched in LREE, LILE, U and Th. The least evolved sample can be modelled using three components: batch melting (15-20% without residual garnet) of wedge peridotite, enriched by partial ( 1-3%) melts of subducted sediments that make up 0.2-0.4% of the source, and slab fluids that contribute 35-85% of the fluid mobile element budget. Sandies from the SEC are in 230Th-238U radioactive equilibrium, suggesting that >350 ka have passed since U-Th differentiation by slab fluids. The Kameni dacites and some of their enclaves, however, have (230Th/232Th) = 0.91-0.99, correlated with (238U/232Th) = 0.91-1.04, dating the time since U-Th differentiation at 147 (+27/-21) ka (1σ). On the basis of comparison with the global dataset for fluid transfer times at arcs built on crust of variable thickness, most of these ages reflect long crustal magma storage times. U-Th WR-mineral isochrons from samples of the SEC and a Kameni dacite give ages of 85 (+22/-19) ka (1σ) and 18 (+19/-16) ka (1σ) respectively, indistinguishable from their eruption ages, suggesting that they date crystallization prior to eruption. Most Kameni dacites have (226Ra/230Th) < 1. This is not due to fluid mobility of Ra, but can be modelled by up to 40% plagioclase fractionation <1 ka prior to their eruption. Using ion microprobe data of trace element traverses across plagioclase crystals from the Kameni dacites and Soufriere, St. Vincent, a model is developed that allows maximum plagioclase crystal residence times to be estimated from incomplete diffusion of Sr profiles. Three out of eight plagioclase phenocrysts have Sr concentration profiles that are not in diffusive equilibrium. For these, the diffusion model is employed to calculate maximum crystal residence times, ranging from 100 to 450 years. This emphasises the importance of distinguishing between crystal residence times and crustal magma storage times, that for Soufriere are thought to h& in excess of 40 ka on basis of U-Th mineral isochron data.