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

Paroxysms at Stromboli are the most violent manifestations of the persistent activity and are related to the emission of small volumes (10 3 –10 5 m 3 ) 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 Fo 88–91 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 K 2 O 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 (Fo 87±0.5 –Fo 83±0.5 ), (2) the variable ratio between melt and gas bubble, and (3) the variability of melt inclusion compositions in both major (CaO/Al 2 O 3 = 1–0.59) and volatile (3.4–1.8 wt % H 2 O, 1582–1017 ppm CO 2 ) 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.