When magmas do not interact: paired Roman-age activity revealed by tephra studies at Stromboli volcano

Detailed stratigraphic and geochemical studies were carried out on remnants of the partially collapsed Pizzo-o-Sopra-la-Fossa (Pizzo) scoria cone, which represents the best viewpoint for observing the present-day activity on top of the Stromboli volcano, Italy. The main outcrop is above the active craters and is strongly altered by acid fumarolic gases; for this reason, the Pizzo activity has previously been poorly studied (for age and composition). We focus mainly on the well-preserved Le Croci fallout succession. We show that the Pizzo activity occurred in the Roman age (similar to 2.4-1.8 ka) and was fed by two compositionally and genetically distinct magma groups, each occupying a separate reservoir. Shoshonitic basalts (Pizzo-SHO) were stored at lower depths than high-K basalts to high-K basaltic andesites (Pizzo-HKCA). Although PizzoSHO and Pizzo-HKCA magmas were affected by multiple mafic magma inputs (with different dynamics as reflected in variable mineral zoning), they did not interact with each other. They were, however, erupted simultaneously suggesting they were emitted by two unconnected conduits and vents located in the summit area. Stromboli historic activity was thus quite different from present-day activity. In the Roman age, the volcano was characterised by paired activity taking place on the summit with similar explosive characteristics, from two coeval vents each fed by different and non-parental magmas. While the Pizzo-SHO magmas still persist in the present-day shallow reservoir, the Pizzo-HKCA magmas could be quiescent and re-activated in the future, possibly from an eccentric vent/fissure.