Plinian vs. caldera-forming eruptions and their controlling factors: the case of Ventotene, Tyrrhenian Sea, central Italy

Among the physico-chemical factors of magmas controlling the dynamics of explosive eruptions, volatile concentration and exsolution depth play relevant roles. The syn-eruptive changes in vesicularity and microlite abundance are linked to decompression degassing, in turn depending on the depth of the magma reservoir. This depth condition is also relevant for caldera-forming eruptions. Major explosive eruptions (VEI 4-6) of Quaternary potassic volcanoes in central Italy categorize into three scenarios: 1) pure Plinian events, with sustained columns that do not evolve into caldera-collapse (e.g., early activity of Vico); 2) "underpressure caldera"-forming events, which shift from initial Plinian phases fed by central conduits to pyroclastic-current (pc) activity and associated lag-breccias erupted from the ring faults during caldera-collapse (e.g., several trachy-phonolitic events of the Roman Province); 3) large mafic ultrapotassic pc events (i.e., Colli Albani) lacking precursor Plinian activity, and linked to "overpressure calderas". This study centers on Ventotene (Pontine Islands, Tyrrhenian Sea), showing shifts from effusive and mild explosive behavior to repeated Plinian events (type 1) and, finally, to a caldera-forming event of type 2. New geological survey and compositional analyses (mineral assemblages, major and trace elements, glass and mineral chemistry, Sr isotopes) of the volcanic successions, and laboratory experiments (volatile solubility, phase relationships and crystallization kinetics), are aimed at assessing the relationships between magma feeder systems and changing eruptive styles. Ventotene and Santo Stefano islet nearby are characterized by potassic volcanics of low-K series, ~0.9-0.3 Ma in age (pending new 40Ar-39Ar datings under acquisition). Ventotene Island is a vestige of a stratovolcano flank, emerging from depths of 600-700 m up to 134 m a.s.l. Its sub-aerial portion consists of K-basalt to shoshonite lava flows and intervening scoria cones, overlain by latitic to trachy-phonolitic pyroclastic deposits (Cala Battaglia, UCB, and Parata Grande, PGT, units). The morphological depression (~3 km across) W of Ventotene is attributed to a caldera-collapse related to PGT. We focus on the contrasting features of UCB and PGT eruptions, as a result of different magma storage conditions and ascent paths. UCB includes repeated Plinian pumice-fall deposits, separated by paleosols. Instead, PGT shows the typical deposit architecture of an underpressure caldera-forming event, i.e.: basal Plinian-fall, welded spatter, lag-breccia, pumice-rich pc's, and hydromagmatic pc's. UCB pumices show high vesicle amounts (>50 vol%), primary analcime microlites and low phenocryst contents (<3 vol%), indicating deep levels of magma fragmentation, in turn linked to deep (>200 MPa) pre-eruptive magma systems, which impeded caldera-collapses. PGT pumices in Plinian fall and pc deposits contain abundant phenocrysts (>10 vol%) and homogeneous phono-trachytic glasses with feldspar microlites, indicating a relatively shallow magma chamber (<200 MPa), prone to caldera-collapse. The high amounts of olivine and clinopyroxene antecrysts in the PGT welded spatter indicate the isothermal depressurization of the peripheral, degassed, mafic portion of the magma chamber at the onset of caldera-collapse. To test the hypothesisof deep, polybaric magma differentiation (leading to pure Plinian events) vs. isobaric differentiation in shallow, sill-like, magma chambers (leading to underpressure caldera-forming events), we explore experimentally different P-T trajectories. Starting from a hydrous shoshonite (3 wt.% of H2O), up to an oversaturated phonolite, a set of stepwise fractional crystallization experiments is run at variable pressures (200-800 MPa) and temperatures ranging from shoshonite near-liquidus conditions (~1100 °C) down to 750 °C