Magma ascent rates and depths of crustal magma reservoirs beneath the Aeolian volcanic Arc (Italy): Inferences from fluid and melt inclusions in xenoliths

Quartz-rich xenoliths in lavas and pyroclasts (basalts to rhyolites) from Alicudi, Salina, Filicudi Vulcano and Stromboli contain abundant fluid and melt inclusions, which bear striking similarities on a regional scale. Two main generations of CO2-rich (+/- H2O or N-2) fluid inclusions are present in quartz grains: early (Type 1) inclusions related to partial melting of the host xenoliths, and Type II inclusions trapped during xenolith ascent in the host magma. Homogenisation temperatures of fluid inclusions show a similar bimodal distribution, corresponding to two distinct density intervals: 0.9-0.5 (Type I) and 0.4-0.1 g/cm(3) (Type II). Early Type I fluid inclusions indicate trapping pressures between 0.6 and 0.3 GPa, which are representative for the levels of partial melting of crustal rocks and xenolith formation. Late Type II fluid inclusions show considerably lower trapping pressures, between 0.17 and 0.05 GPa, indicative for shallow magma rest and accumulation during ascent to the surface. The present study indicates ponding of mantle-derived magmas at lower crustal depths and at shallow levels. Deep crustal reservoirs (25-12 km) are present beneath the whole archipelago and indicate that substantial melting processes occur at middle to lower-crustal conditions. The main stages of anatexis, occur at these depths approximating the Moho, and may have been induced by a regional heating during underplating and intrusion of the continental crust by mantle-derived basaltic magmas. The shallow magma chambers (6-4 km) are fed by the deep-sited crustal reservoirs and are the site of evolutionary processes such as fractional crystallisation and mixing.