Consequences of CO2-magma interaction for explosive volcanism at Colli Albani (Italy)

Abstract:

Colli Albani is a mafic-alkaline volcanic complex that repeatedly produced large caldera forming eruption of up to 63 km³ (DRE). The volcano is located just 30 km from the densely populated city of Rome. Mafic and alkaline magmas are rarely associated with such large volume explosive events. The cause of this unusual explosivity is thought to be related to the addition of CO₂ resulting from the interaction with carbonates. However, whether the CO₂ is supplied by shallow crustal carbonates or by carbonatic material at the top of the subducted Adriatic plate remains controversial. We measured the volatile content of melt inclusions hosted in clinopyroxene and find that several contain >2 wt. % CO₂ with values as high as to 3.5 wt. % CO₂ and 3.9 wt. % H₂O. CO₂ solubility studies indicate that such values are not compatible with shallow magma storage. Additionally, clinopyroxene phenocrysts have high magnesium number (up to 0.93) and Cr₂O₃ content, which suggest equilibrium with the mantle. We performed thermo-barometry of the clinopyroxene phenocrysts using a random forest machine learning approach and found pressures and temperatures of up to 16 kbar and 1250 °C. Such high pressure and temperature clinopyroxenes are particularly abundant at the base of the ignimbrite outcrop. We suggest that mafic and CO₂-rich magma rapidly ascends from the mantle, destabilizing a shallow crustal magma reservoir and triggering an eruption. Magma in the shallow crust surely interacts with the host carbonates, however, we suggest that the high explosivity of this system is better explained by the addition of large amounts of CO₂ to the magma in the mantle, which allow for its fast ascent through the lithosphere.