Stromboli volcano (Aeolian Archipelago, Italy): An open window on the deep-feeding system of a steady state basaltic volcano

[1] Paroxysms at Stromboli are the most violent manifestations of the persistent activity and are related to the emission of small volumes (103–105 m3) of nearly aphyric HK-basaltic pumices. They offer the exceptional opportunity to detail the mixing-crystallization-degassing processes that occur in a steady state basaltic arc volcano. We present mineralogy, major, volatile, and trace element geochemistry of olivine-hosted melt inclusions of these pumices. In all the paroxysms, melt inclusions hosted in olivines Fo88–91 have recorded the parental melts rich in CaO (up to 14.5 wt %) but low in FeO (6–7 wt %). They demonstrate recurrent variations in the K2O content (1.6–1.3 wt %) and S/Cl ratios (1.2–0.8) of the melts that entered the deep system. Dynamic magma mixing between melts slightly distinct by their degree of evolution, rapid crystallization, and entrapment of gas-oversaturated melts during decompression are indicated by (1) the high density of irregular, clear melt inclusions, and embayments in homogeneous olivines (Fo87±0.5–Fo83±0.5), (2) the variable ratio between melt and gas bubble, and (3) the variability of melt inclusion compositions in both major (CaO/Al2O3 = 1–0.59) and volatile (3.4–1.8 wt % H2O, 1582–1017 ppm CO2) elements. FeO-rich melt inclusions in patchy, reversely zoned olivines also demonstrate interactions between ascending melt blobs and inherited olivine crystals. We propose a model involving a vertically extended dike-like system, where magmas progress and differentiate. On the basis of olivine growth rate calculations the volatile-rich magma blobs may ascend within few hours to few tenths of hours. Finally, we propose that sulfur degassing is possibly initiated during the early stage of magma differentiation.