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

ABSTRACT

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 continental arc 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 δ18O_cpx = + 5.37 to + 6.20 ‰ SMOW, corresponding to δ18O_melt = + 5.6 to + 6.4 ‰ and display an overall, but poorly defined, positive correlation with ⁸⁷Sr/⁸⁶Sr and MgO and a rough negative correlation with SiO₂. Basalts exhibit the highest δ18O_cpx values for the volcanic series studied. Together, major, trace element and isotopic data indicate complex, multistage polybaric evolutionary processes for the Filicudi magmas. A process of fractional crystallisation, accompanied by variable degrees of crustal assimilation, best explains the isotopic and petrological data of the Filicudi volcanics. Evolution processes are inferred to have occurred in distinct magma reservoirs sited at different depths. Magma contamination affected basalts more extensively than andesitic and dacitic magmas. This was a consequence of the higher temperature and lower viscosity of mafic melts that were able to assimilate higher amounts of crustal wall rocks. Therefore, andesites and dacites preserved stable and radiogenic isotopic compositions more closely than basalts. Combined trace element and isotopic data suggest that primitive magmas at Filicudi were generated in a heterogeneous and metasomatised mantle source, which underwent contamination by slab-derived fluids. When compared with the nearby islands, isotopic variations of Filicudi rocks resemble more closely those found at Alicudi than at Salina and Vulcano. This suggests that the same kind of evolutionary processes occurred in the western island of Filicudi and Alicudi, whereas at Salina and Vulcano, in the central arc, magmas typically evolved by AFC processes, generating derivative melts that display higher Sr-O isotope compositions compared to parent basalts. These contrasting styles of magma evolution may be related to different structures of the plumbing systems of volcanic islands that were constructed in different sectors of the arc.