Application of paleomagnetic methods on two variably welded peralkaline fallout deposits from Pantelleria Island (Sicily Strait): inferences on emplacement temperature and welding

Abstract

One of the major volcanological issues is to outline the eruptive and depositional mechanisms of explosive eruptions and to constrain the emplacement temperature (Temp) of the associated pyroclastic deposits for the inferences on the thermal evolution, welding and rheomorphism, before cooling to the glass transition temperature (Tg). Emplacement temperature of pyroclastic deposits, in particular ignimbrites, can be constrained using paleomagnetic methods on lithic clasts entrained during eruptive climax. At Pantelleria island, a paleomagnetic study was carried out on the Green Tuff welded and rheomorphic ignimbrite [Scarani et al., 2023], but there is limited understanding of less explosive inter-ignimbritic eruptions. Here we provide paleomagnetic data of two variably welded fallout deposits belonging to the old local centres of Cala dell’Altura (pantelleritic) and Cala delle Giache (comenditic), in the attempt to ascertain any possible relationship of welding degree of peralkaline airfalls with the emplacement temperature recorded by lithic clasts. Thermal demagnetization of lithic clasts does not show evident relationships with welding in the Cala dell’Altura fallout, indicating that unwelded and welded layers were emplaced at the same temperature range between the TCurie of magnetite (580 °C) and T pre-eruptive (700°C for pantelleritic magma, 800 °C for comenditic), thus suggesting poorly efficient cooling (low column height or high deposition rate). On the contrary the Cala Giache fallout, characterized by some initial phreatomagmatic episodes, shows an evident bimodality, with lithics in the welded layers emplaced at high temperature (580-800°C) and those in the unwelded layers at lower temperature, pointing out to a more efficient cooling, possibly due to early magma-water interaction and/or lower deposition rate. Post-depositional magnetic disturbance occurred in proximity of some large (30 cm in length) lithics, deforming very local (cm-scale) geomagnetic field and affecting the magnetic orientation of smaller lithics nearby. Secondarily, we detected post-depositional tilting of few lithics due to sliding along the unconsolidated matrix of the unwelded fallouts.