A study of melt inclusions at Vulcano (Aeolian Islands, Italy): insights on the primitive magmas and on the volcanic feeding system

This work presents the results of a microthermometric and EPMA-SIMS study o f melt inclusions in phenocrysts of rocks of the shoshonitic eruptive compl ex of Vulcano (Aeolian Islands, Italy). Different primitive magmas related to two different evolutionary series, an older one (50-25 ka) and a younger one (15 ka to 1890 A.D.), were identified as melt inclusions in olivine Fo (88-91) crystals. Both are characterized by high Ca/Al ratio and present ve ry similar Rb/Sr, B/Be and patterns of trace elements, with Nb and Ti anoma lies typical of a subduction zone. The two basalts present the same tempera ture of crystallization (1180 +/- 20 degrees C) and similar volatile abunda nces. The H2O, S and Cl contents are relatively high, whereas magmatic CO2 concentrations are very low, probably due to CO2 loss before low-pressure c rystallization and entrapment of melt inclusions. The mineral chemistry of the basaltic assemblages and the high Ca/Al ratio of melt inclusions indica te an origin from a depleted, metasomatized clinopyroxene-rich peridotitic mantle. The younger primitive melt is characterized with respect to the old er one by higher K2O and incompatible clement abundances, by lower Zr/Nb an d La/Nb, and by higher Ba/Rb and LREE enrichment. A different degree of par tial melting of the same source can explain the chemical differences betwee n the two magmas. However, some anomalies in Sr, Rb and K contents suggest either a slightly different source for the two magmas or differing extents of crustal contamination. Low-pressure degassing and cooling of the basalti c magmas produce shoshonitic liquids. The melt inclusions indicate evolutio nary paths via fractional crystallization, leading to trachytic composition s during the older activity and to rhyolitic compositions during the recent one. The bulk-rock compositions record a more complex history than do the melt inclusions, due to the syneruptive mixing processes commonly affecting the magmas erupted at Vulcano. The composition and temperature data on mel t inclusions suggest that in the older period of activity several shallow m agmatic reservoirs existed; in the younger one a relatively homogeneous fee ding system is active. The shallow magmatic reservoir feeding the recent er uptive activity probably has a vertical configuration, with basaltic magma in the deeper zones and differentiated magmas in shallower, low-volume, dik e-like reservoirs.