Lithium, Beryllium and Boron in high-K Rhyolites from Lesbos Island, Greece

Abstract

This study investigates the behaviour of Li, Be, and B in volcanic rocks as part of the subduction cycle. The partitioning of Be and B during melt evolution and volcanic processes is examined and the volcanic setting is characterized using petrological,geochemical, and isotopic tools. The geological setting of Lesbos Island within the Aegean-Anatolian area is part of a complex system of convergence and collision of the Eurasian, African and Arabian plates. Several conflicting models for the tectonic setting of the Miocene volcanism in the Aegean exist, with the quintessential question being, if the volcanic rocks are related to subduction or to crustal extension. Lesbos Island lies in the northeastern Aegean Sea and consists of thick Miocene volcanic deposits, which overlie a metamorphic basement. The object of this study are two vertical sample profiles (Profile A and Profile B) through the Early Miocene Polychnitos Ignimbrite, an over 100m thick ignimbrite deposit in the centre of the island. The ignimbrite contains large portions of pristine glass and therefore allows the direct observation of the quenched melt. The typical composition of the pristine high-K rhyolitic glass is SiO2 69wt%, K2O 7wt%, H2O 4wt%. The magma source was characterized using the B isotopic composition of the volcanic glass with relatively high d11B (-2.30±1.28‰ to -3.22±1.07‰), fluidmobile trace element contents and ratios (B=101µg/g, B/Nb=3.90) in comparison to upper continental crust, and results from Sr, Nd, and Pb isotope analyses.The elevated d11B values demonstrate, that the magma sources for samples from the Profile A and Profile B have been enriched by slab-derived fluids, while crustal material was not assimilated. Magma chamber P-T conditions for samples from Profile A were investigated using a combination of six methods, feldspar thermobarometry, accessory mineral saturation thermometry, and H2O solubility in rhyolitic melts. Pressures in the magma chamber were between 75MPa and 150MPa (corresponding magma chamber depth 2.6km to 5.3km) and temperatures between 815oC and 848oC. Equilibrium between melt and phenocrysts was reached under these conditions. Taking advantage of the high amount of pristine glass, partition coefficients were determined for Be and B between glass, and plagioclase and alkali feldspar (DBe Pl/glass=0.36 (An20); DBe Pl/glass=0.43 (An28); DB Pl/glass=0.009; DBe Akf/glass=0.06;DB Akf/glass=0.005) and establish a well-founded data set for high-silica magmatic systems. A positive correlation between the An content of the plagioclase and DBe Pl/glass was confirmed. Using selected trace element contents in the volcanic glass (Sr, Ba, Pb) and simple fractionation models it is shown, that samples from the two vertical profiles did not have a common parent melt and that neither melt evolved from the other by fractional crystallisation and / or crystal assimilation. A separate origin of samples from the two vertical profiles was shown by the significantly different Sr isotopic composition of contained plagioclase phenocrysts (Profile A 87Sr/86Sr = 0.70827(5); Profile B 87Sr/86Sr = 0.70798(7)). However, both melts have undergone a substantial amount of fractional crystallisation, as expected for rhyolitic melts, and as shown through enrichment of Be (5.2µg=g), Nd (46µg=g), and Nb (25µg=g) of the volcanic glass in comparison to upper continental crust. The two vertical profiles, Profile A and Profile B, are demonstrated to stem from two separate ignimbrite units within the Polychnitos Ignimbrite, and to have originated from separate volcanic vents, separate volcanic plumbing systems, and ultimately from separate magma source areas within the mantle. The two magmas have evolved separately, yet in a similar way: prior to melting both source areas have been subject to metasomatism from slab fluids, both magmas have undergone a substantial amount of fractional crystallisation without assimilation of ancient crustal material, and both melts have assimilated plagioclase xenocrysts from a cogenetic magma, probably from older volcanics within the same volcanic plumbing system. Despite substantial petrographic differences throughout both vertical profiles, especially in Profile A, each profile is geochemically homogeneous throughout glass and phenocrysts for all analysed elements, with the exception of lithium. Evidence for significant Li diffusion was detected in glass and minerals, and together with high-temperature devitrification, was used to develop a model combining late degassing (after deposition), Li diffusion and hydrothermal “cooking” of the matrix. The systematics of lithium, beryllium, and boron were investigated within a wellcharacterised petrological and geochemical background of mantle signatures, melt evolution, magma chamber conditions and post-depositional processes.