Oxygen Isotopic Variations in the Clinopyroxene from the Filicudi Volcanic Rocks (Aeolian Islands, Italy): Implications for Open-System Magma Evolution

摘要

Oxygen isotope data are reported for clinopyroxene phenocrysts from volcanic rock samples from Filicudi island,Italy, with the aim of investigating mechanisms of magma evolution and mantle source characteristics in a continentalarc volcano. Filicudi rocks range from calc-alkaline basalt to high-K andesite and dacite. Mafic rocks have MgO≤ 6-7 wt%, Mg# ≤ 60, Ni < 50 ppm and Cr < 200 ppm, suggesting they represent evolved melts from more primitive mantle-derived parents. Variations of Sr-Nd isotope ratios against silica and MgO suggest magma evolution under opensystem.Oxygen isotope ratios on clinopyroxene phenocrysts from representative rocks are in the range δ18Ocpx = + 5.37 to+ 6.20 ‰ SMOW, corresponding to δ18Omelt = + 5.6 to + 6.4 ‰ and display an overall, but poorly defined, positive correlationwith 87Sr/86Sr and MgO and a rough negative correlation with SiO2. Basalts exhibit the highest δ18Ocpx values forthe volcanic series studied. Together, major, trace element and isotopic data indicate complex, multistage polybaric evolutionaryprocesses for the Filicudi magmas. A process of fractional crystallisation, accompanied by variable degrees ofcrustal assimilation, best explains the isotopic and petrological data of the Filicudi volcanics. Evolution processes are inferredto have occurred in distinct magma reservoirs sited at different depths. Magma contamination affected basalts moreextensively than andesitic and dacitic magmas. This was a consequence of the higher temperature and lower viscosity ofmafic melts that were able to assimilate higher amounts of crustal wall rocks. Therefore, andesites and dacites preservedstable and radiogenic isotopic compositions more closely than basalts. Combined trace element and isotopic data suggestthat primitive magmas at Filicudi were generated in a heterogeneous and metasomatised mantle source, which underwentcontamination by slab-derived fluids. When compared with the nearby islands, isotopic variations of Filicudi rocks resemblemore closely those found at Alicudi than at Salina and Vulcano. This suggests that the same kind of evolutionaryprocesses occurred in the western island of Filicudi and Alicudi, whereas at Salina and Vulcano, in the central arc, magmastypically evolved by AFC processes, generating derivative melts that display higher Sr-O isotope compositions comparedto parent basalts. These contrasting styles of magma evolution may be related to different structures of the plumbingsystems of volcanic islands that were constructed in different sectors of the arc.