The crystal cargo provides a chronicle of pre-caldera dynamics in mafic volcanic systems: insights from Colli Albani.

Abstract

Understanding the processes leading up to caldera-forming eruptions is essential for identifying precursory signals of catastrophic events. While these phenomena have been extensively studied in silicic systems, mafic volcanoes present unique challenges. Indeed, the high eruptive temperatures of mafic magmas might imply short storage in the cold upper crust and, thus, short periods of unrest preceding eruption, which could challenge our capacity to mitigate the impact of an imminent event. In this study, we present new textural data, major- and trace-element analyses, and quantitative trace-element maps of the crystal cargo from an effusive to mildly explosive sequence (the Fontana Centogocce Formation) and the subsequent caldera-forming phase (the Villa Senni Formation) at the Colli Albani volcano in Italy. By integrating well-established and data-driven approaches, we constrain the processes and dynamics that drive the transition from mildly explosive to highly explosive activity in the studied magmatic sequences. Our findings reveal that the effusive to mildly explosive eruptions preceding the caldera-forming event were fed by multiple magma reservoirs emplaced at shallow crustal levels ( $\sim$ 1-4 kbar). Following a quiescent period recorded by a paleosol, more primitive magma rose directly from the mantle and accumulated at multiple crustal levels. The ascent of one of these magma pulses ultimately triggered Colli Albani's last caldera-forming eruption.