CO2-DRIVEN LARGE MAFIC EXPLOSIVE ERUPTIONS: A CASE STUDY FROM THE COLLI ALBANI VOLCANIC DISTRICT (ITALY)

Magma degassing is a critical process that play a major role in volcanic eruption style. Despite what generally thought not all volatiles emitted from volcanoes originate from a magmatic source, i.e. they exsolve from their host magma during ascent to the surface. Volcanoes rooted on thick sedimentary crust, indeed, have the potential to re-mobilise crustal volatiles. In particular, volcanoes emplaced in carbonate crust may re-mobilise a large amount of CO2 that significantly contributes towards the volcanic volatile budget and can play a critical role in enhancing the explosivity of the eruptions. Generally, the intensity and magnitude of explosive volcanic activity increase parallel to SiO2 content: i.e., relatively low-viscosity mafic magmas feed effusive or mildly explosive eruptions, whereas high-viscosity silicic magmas feed large explosive eruptions such as plinian and pyroclastic-flow forming ones. Pyroclastic-flow eruptions from the Colli Albani ultra-potassic volcanic district (Italy), one of the best examples of magmatic plumbing system emplaced within a thick carbonate sequence, however, represent a striking exception on a global scale. The juvenile fraction in pyroclastic flow deposits, which attain individual volumes in the order of tens of km3, is K-foiditic in composition, with SiO2 contents as low as 42 wt%, i.e. even much lower than those typical of basalts. We discuss the driving mechanisms for large mafic explosive eruptions based on the reconstruction of the pre-eruptive scenario and event dynamics of the most powerful Colli Albani eruption: the ~456 ka Pozzolane Rosse (PR) eruption. The PR eruptive succession begins with the effusion of a wide lava plateau from a peripheral vent, continues with a subplinian to moderate plinian eruption and ends with a single massive, poorly sorted pyroclastic flow body. We suggest that the addition of CO2 to the magma, due to carbonate assimilation from country rocks, is the major factor controlling explosivity. High CO2 activity in the volatile component, coupled with magma depressurisation, produced extensive leucite crystallisation, resulting into a dramatic increase of magma viscosity and volatile pressurisation and in turn into changing eruptive dynamics from early effusive to highly explosive at the PR eruption climax. The PR event may thus be regarded as the CO2-dominated end-member of a wide spectrum of volatile conditions controlling magma chamber processes, as well as eruptive and emplacement dynamics. The present case study provides evidence that the addition of free CO2 from entrained country rocks may drive mafic H2O-undersaturated magmas toward anomalously high-intensity explosive behaviour, as typical of silicic, H2O-dominated magmas.