Timescales of destructive plate margin magmatism: new insights from Santorini, Aegean volcanic arc

Abstract

The timescales of element transfer at volcanic arcs provide important insights into magma generation, movement and storage beneath destructive margin volcanoes. Here we present major, trace and Sr-, Nd- and U-series isotope data on <200 ka old samples from Santorini in the Aegean volcanic arc as a case study of a hazardous destructive margin volcano. Samples range from low-K calc-alkaline basalts to high-K rhyolites, are enriched in light rare earth elements and fluid mobile trace elements, and have negative Ta, Nb and Ti anomalies characteristic of arc magmatism. 87Sr/ 86Sr ratios range from 0.7035 to 0.7054 and are negatively correlated with 143Nd/ 144Nd ratios, ranging from 0.51285 to 0.51263. Lavas from the second eruptive cycle (∼180 ka–∼3.6 ka) are in U–Th radioactive equilibrium, and a whole-rock, plagioclase, magnetite U–Th mineral isochron from a 67±9 ka dacite yields an age of 85 (+22/−19) ka (1σ). Samples from the Kameni islands (46 AD–1950) show small but significant uranium excesses, with ( 238U/ 232Th)=0.91–1.04 and ( 230Th/ 232Th)=0.91–0.97, suggesting that the time since U–Th differentiation is ≤147 (+27/−21) ky (1σ). A U–Th mineral isochron from the 1940 Kameni dacite yields an age indistinguishable from its eruption age. The Kameni samples show variable Ra depletion, with ( 226Ra/ 230Th)=0.92–0.99. The range in Sr and Nd isotope data reflects progressive crustal assimilation. The combined trace element and isotope data are consistent with a three-component source model involving mantle wedge, sediment melts and slab-derived fluids. While initial 238U-excesses in the second eruptive cycle have decayed, implying slab-derived fluid addition >180 ky prior to eruption, for the Kameni lavas fluid addition to the wedge is ≤150 ky. U–Th mineral isochrons indicate that crystal residence times in the subvolcanic plumbing system are short. Fractional crystallisation less than ∼1 ky prior to eruption of the Kameni dacites is suggested by ( 226Ra/ 230Th)<1. The combined data imply that timescales of magmatic differentiation in crustal reservoirs are short, that transfer time of the slab-fluid signature through the mantle wedge varies through time.