Sr, Nd, and Pb isotope evidence for open system evolution at Vulcano, Aeolian Arc, Italy

Abstract

In this paper isotopic (Sr, Nd, and Pb) and geochemical data are discussed with the aim of developing a model for the evolution of the magmatic system of Vulcano (Aeolian Arc, Italy). The magmatism at Vulcano is shoshonitic in affinity. The Vulcano products display, throughout the stratigraphic sequence, repeated variations in the isotope ratios ( 87Sr/ 86Sr =0.70426–0.70524; 143Nd/ 144Nd =0.51254–0.51273; 206Pb/ 204Pb =19.287–19.548; 207Pb/ 204Pb =15.616–15.865 and 208Pb/ 204Pb =39.132–39.818). The isotopic variations are correlated with the degree of evolution of the rocks, the most crustal compositions occurring in the most differentiated rocks, and are interpreted as produced by AFC (assimilation+fractional crystallization). The chemical and isotopic data suggest that in the evolution of the volcanic rocks older than 25 ka (Vulcano Primordiale and opening phases of `Caldera della Fossa'), basaltic melts underwent fractional crystallization and were contaminated by crust material that was isotopically similar to that of the lower crust. This occurred in different shallow magma chambers, and the melts evolved to mainly shoshonitic compositions. The shoshonitic magmas further evolved to rhyolites through AFC within chambers in the upper crust, which fed the Mastro Minico and Lentia complexes (25–15 ka). In the most recent phase of activity, after the collapse of the Lentia complex (post-15 ka), shoshonitic and latitic magmas were enriched in alkalies and hygromagmatophile elements with respect to the older rocks. This change in composition may be interpreted as due to a change in the primary magmas, as a result of a lower degree of partial melting of the mantle source compared with the older products. Alternatively, this may occur through magma evolution by AFCB processes (assimilation during fractional crystallization+tapping+batch replenishment) in a shallow magma chamber. Shoshonites and latites evolved further through AFC to rhyolitic compositions, in small, very shallow magma traps emplaced in upper crust material. Isotopic modelling indicates that syneruptive mixing, together with AFC, determines the isotopic complexity of the La Fossa products.