Open-closed conduit transitions regulate the large magnitude explosive eruptions of Petrazza, PaleoStromboli I (Stromboli, Italy).

A multi-disciplinary approach of volcano-stratigraphy, petrology and geochemistry has shed light on the pre-eruptive processes, the eruptive triggering, behaviour and the architecture of the magma plumbing system during the explosive cycle of Petrazza at ca. 77-75 ka (PaleoStromboli I eruptive epoch, Stromboli). This was the largest magnitude eruptive cycle in Stromboli and one of the largest of the entire Aeolian archipelago, able to produce Vulcanian to sub-Plinian/Plinian phases with distal deposits found in the tephrostratigraphic record of the Tyrrhenian sea and surroundings. Our study highlighted that, differently from the present-day activity, the large magnitude Petrazza eruptive cycle could be attributed to phases of closed-system conditions, as also testified by the in equilibrium presence of amphibole, indicative of a "steady-state" magmatic status of the system. The explosive activity is then attributed to strong depressurization underwent by the plumbing system due to the cyclic closure/opening of the shallow conduit, possibly also in association with lateral collapse events. As shown by textural and compositional studies on plagioclase crystals, this decompression was also able to recall amphibole bearing mafic magma from the deep portion of the plumbing system (5-15 km of depth).