High vs Low Pressure Magma Chambers at Ventotene Volcano (Tyrrhenian Sea, Central Italy): Implication for Pure Plinian vs Caldera-Forming Eruptions

Abstract Eruptive styles are influenced by multiple factors, including magma chamber depth. At Ventotene volcano (Pontian Islands, Tyrrhenian Sea, central Italy), a transition at ~300 ka is observed from repeated ‘pure Plinian’ eruptions—i.e. without associated caldera collapse (Cala di Battaglia eruptions, UCB)—to a caldera-forming event (Parata Grande eruption, PGT). In the latter eruption, the initial Plinian phase is followed by the roof collapse and widespread pyroclastic currents. We characterize the pre-eruptive magmatic systems of Ventotene—defined as reservoirs immediately preceding eruption and directly feeding syn-eruptive exsolution and fragmentation—through integrated analyses of juvenile (pumice and scoria) clasts, including texture, mineral and bulk-rock chemistry, 87Sr/86Sr isotopic ratios, and experimental phase equilibria. These data provide new insights into contrasting deep vs shallow pre-eruptive magmatic systems that fed pure Plinian and caldera-forming eruptions, respectively, at Ventotene. UCB pumice clasts contain low phenocryst amounts (<3 vol %) and primary analcime microcrysts, whose crystallization is consistent with PH2O conditions higher than 150 MPa, as shown by experimental runs on Ventotene trachyte at PH2O = 150 and 600 MPa. The presence of analcime constrains the UCB pre-eruptive magmatic systems to the deep metapelitic basement, also indicated by higher 87Sr/86Sr ratios. In contrast, PGT juvenile clasts—from Plinian fall, welded spatter, lag breccia, pumice-rich, and hydromagmatic pyroclastic current deposits—contain abundant phenocrysts and antecrysts (10–25 vol %), while analcime is absent. This latter textural feature, coupled with the lower H2O content in glasses and clinopyroxene barometry, points to a shallow (P ≤ 180 MPa) PGT pre-eruptive magmatic system. Notable differences between phonolitic–trachytic groundmass glasses and shoshonitic to tephri-phonolitic/latitic bulk compositions reflect the occurrence of olivine and clinopyroxene antecrysts in the PGT pre-eruptive magmatic system. The presence of centimeter-sized clinopyroxenes lacking resorption textures suggests that this system was short lived. Consistently, the longevity of the PGT pre-eruptive magmatic system is estimated to be <2 years, based on experimentally calibrated clinopyroxene and amphibole growth rates and settling distances of ~1-cm-sized antecrysts. Homogeneous glass compositions but variable phenocryst assemblages suggest a crystal-zoned pre-eruptive magmatic system with antecryst-depleted (D), enriched (E), and mush (M) zones. The initial Plinian phase tapped the D zone, followed by spatter deposits rich in olivine and clinopyroxene antecrysts from the E zone, concurrent with caldera collapse onset. The low water content in these glasses indicates magma outgassing. This evolution is consistent with a transition from a central conduit (sustaining a Plinian column) to a multi-vent (fissural) system along ring faults, feeding spatter-, lithic-, and pumice-rich pyroclastic currents. We propose that pure Plinian scenarios at Ventotene—and likely in similar volcanic systems—are driven by polybaric differentiation of a magma batch ascending from depth and/or pre-eruptive storage in deep reservoirs with high roof aspect ratios. In contrast, isobaric differentiation in shallow, sill-like pre-eruptive magmatic systems with low roof aspect ratios favors caldera-forming eruptions. Here, decompression from initial Plinian magma withdrawal induces roof collapse, a hallmark of underpressure caldera scenarios. This has implications for active caldera systems such as Ischia and Campi Flegrei, which share similar magma compositions and eruptive style shifts—from early pure Plinian to later caldera-forming activity—throughout their histories.