OXYGEN ISOTOPE CONSTRAINTS ON THE CRUSTAL CONTRIBUTION TO THE SUBDUCTION-RELATED MAGMATISM OF THE AEOLIAN-ISLANDS, SOUTHERN ITALY

Oxygen isotope data are presented for 37 samples of subduction-related lavas from the Aeolian Islands, southern Italy, that were characterised for radiogenic isotopes (Sr, Nd, Pb) and major- and trace-element abundances in previous studies. The samples, which were all erupted within the past 500,000 years, span full compositional range of Aeolian magmatism from the calc-alkaline basalt-dacite sequence of Salina to the shoshonites of Stromboli and the potassic leucite-bearing basalt to rhyolite series of Vulcano, Whole-rock delta-O-18 values range from + 6.3 parts per thousand to 8.5 parts per thousand, but in some cases (14 samples) O-18 enrichment by post-crystallization hydration and low-temperature alteration is suspected, and an empirical correction for excess water yield a primary magmatic delta-O-18 range of + 6.1 parts per thousand to + 8.5 parts per thousand. Variations in delta-O-18 exhibit a compositional dependence, increased delta-O-18 values characterize the evolved samples of the various magmatic series, and delta-O-18 - SiO2 trends are steeper than those predicted for closed system fractional crystallization. In general, O isotope variations are accompanied by only small shifts in radiogenic isotope ratios and, while weak correlations between delta-O-18 and Sr-87/Sr-86 etc., are observed within individual fractionation series, it is apparent that these radiogenic isotope variations are much smaller than the ranges for the whole data set. A two-stage model of magma genesis is proposed in which hybridization of the mantle by the introduction of subducted sediments gave rise to a range of radiogenic isotope ratios at fairly constant delta-O-18. Subsequently, magmas derived from the heterogeneous mantle evolved within crustal magma chambers by AFC, involving assimilation of O-18-rich crust, and giving rise to evolved magmas with high delta-O-18, decreased Pb-206/Pb-204, but with only slightly higher Sr-87/Sr-86 slightly lower Nd-143/Nd-144.