Unravelling the Ancient (> 46 ka) Volcanostratigraphy of Pantelleria Island (Sicily Strait, Italy) by Single-Grain 40Ar/39Ar Dating

Peralkaline magmatism is frequently associated with highly explosive volcanic activity, making the precise reconstruction of eruptive cycles essential for volcanic hazard assessment, particularly in densely populated magmatic districts such as Pantelleria Island.Located within the Sicily Strait Rift Zone (Italy), Pantelleria represents the subaerial portion of a large, active, composite Quaternary volcanic complex, distinguished as the type locality for pantellerite, an iron-rich, peralkaline rhyolite. Subaerial volcanism has been persistent on the island for over 320 kyr, characterised by at least nine ignimbrite-forming events, linked to a minimum of two caldera collapses, interspersed with several less explosive to effusive eruptions from minor eruptive centres with local extents. We carried out radiometric analyses employing the 40Ar/39Ar method to refine the island’s stratigraphic framework, focusing on both ignimbrite deposits and the early eruptive cycles. Following precise manual selection, alkali feldspar separates were neutron-irradiated at the Oregon State TRIGA Reactor, alongside sanidine from the Fish Canyon Tuff (28.305 ± 0.036 Ma), which served as a flux monitor. Argon isotopic ratios were measured using a state-of-the-art mass spectrometer at the Institut des Sciences de la Terre d’Orléans (CNRS-ISTO).Although the last seven ignimbrite units have been well constrained between 187 and 46 ka by 40Ar/39Ar dating, the eruptive ages of the two considered earliest ignimbrite deposits remained poorly constrained or entirely unknown. Specifically, the Zinedi Fm. was dated at ~ 189 ka using the 40K/40Ar method, even though with a considerable margin of error and undefined internal systematics. The Pozzolana Fm. remained undated, inferred to have been emplaced between 189 and 128 ka based on stratigraphic relationships. Our 40Ar/39Ar data allowed the precise position of these two deposits in the stratigraphic record, with the Zinedi Fm. yielding a Weighted Mean Age (WMA) of 178.1 ± 0.8 ka (Mean Square of Weighted Deviates, MSWD = 1.45, 1σ), and the Pozzolana Fm. exhibiting a WMA of 156.5 ± 1.1 ka (MSWD = 1.77, 1σ). As a result, the Polacca Fm. (~ 187 ka), the most voluminous ignimbrite on the island with an estimated onshore Dense Rock Equivalent (DRE) of 0.64 km³, is now recognised as the oldest ignimbrite of the eruptive cyclicity.Considering the previously established eruptive ages, chemical analyses of the ignimbrites revealed a general decline in peralkalinity and the rate of magmatic evolution, alongside a reduction in erupted magma volumes. The only exception to this trend was the most recent ignimbrite unit, the Green Tuff Fm. (~ 46 ka), which showed a slight increase in these parameters. Our new data not only refine the stratigraphic sequence of major explosive eruptions on Pantelleria, but also improve our understanding of temporal trends. Both geochemical and morphometric data exhibit variability over time, providing new insights into the island's magmatic plumbing system and the dynamics of eruption and emplacement.Beyond clarifying the chronology of the major explosive events, our new data shed light on the onset, spatial migration, and evolution of subaerial volcanism at Pantelleria.The Pantelleria’s eruptive history is divided into ignimbrite periods and inter-ignimbrite periods for simplicity. The latter are characterised by the activation of small volcanic centres distributed throughout the island, which have produced lava flows and fallout deposits predominantly pantelleritic in composition. Mafic volcanism is confined exclusively to the northern sector of the island, representing only about 5% of the exposed lithology.The eruptive activity following the emplacement of the most recent Green Tuff ignimbrite has been extensively documented in the scientific literature. 40Ar/39Ar dating has particularly constrained the eruptive period to < 20 ka, identifying the most recent subaerial eruption as that of Cuddia Gallo in the northern sector of the island (~ 7 ka). Conversely, the activity preceding the Green Tuff eruption remained poorly investigated. The subaerial eruptive onset had previously been assessed approximately between 303 and 324 ka in the southwestern sector of the island, between Punta di San Gaetano and Cuddia di Scauri, based on two 40K/40Ar ages with large analytical uncertainties (± 11-72 ka). Our new 40Ar/39Ar results suggest that subaerial volcanism first occurred slightly further southeast at Cala delle Capre, where the lava flow at the base of the stratigraphic sequence provided an accurate WMA of 326.5 ± 0.9 ka (MSWD = 1.52, 1σ). Subsequently, volcanism migrated to the southeastern sector (up to ~ 200 ka at Punta del Formaggio), until the activation of the Punta Pozzolana centre to the northeast (~ 189 ka), predating or simultaneously active with the earliest ignimbrite event. The following eruptive activity was characterised by several less explosive to effusive eruptions occurring between the major explosive events across the entire island, with a northward migration during the most recent stages, culminating in the latest submarine basaltic effusion ~ 4 km northwest of Pantelleria in 1891 C.E. Overall, these findings significantly improve the geochronological framework for Pantelleria, enhancing our understanding of stratigraphic sequences, lateral correlations, and eruption dynamics, ultimately providing new insights into the rates and timing of volcanic activity on the island.