The Onano eruption (Latera volcano, Central Italy): an example of magma mixing/mingling as dominant process in a caldera-forming eruption

The Onano eruption (~ 0.17 Ma) is the second to last caldera-forming eruption of the Latera volcano, in the northernmost sector of the Roman Comagmatic Province (Latium, Italy). The stratigraphic sequence, from base to top, includes ash and pumice-rich flow deposits, spatter-rich flow deposits and lag breccias associated with ash-rich flow deposits. By combining major and trace element compositions of the bulk rocks, matrix glasses and minerals of juvenile components from the different depositional units, we reconstruct the pre-eruptive evolution of the magma chamber and the syn-eruptive magma dynamics. Juvenile clasts with heterogeneous glass composition and/or mineral assemblage are a ubiquitous feature of the Onano eruption. The products cover a large compositional range from phonotephrite to phonolite. They are crystal poor, with felsic paragenesis associated to Mg-rich olivine ( Fo82-90) and diopside ( Fs4-7), these last not in equilibrium with the erupted melts. The mafic mineral assemblage suggests that the pre-eruption magma reservoir was periodically perturbed by the arrival of a primitive magma carrying on Mg-rich minerals and/or remobilizing a mafic crystal mush at the bottom of the reservoir. According to the results obtained from both rhyolite-MELTS and Rayleigh crystal fractionation modeling, we infer that the system evolved from phonotephrite to phonolite both via crystal fractionation and magma mixing between the two end members. Crystallization mostly proceeded at the wall of the reservoir, while magma mixing in the middle zones of the chamber generated the intermediate tephriphonolitic melts. The pre-eruptive chemical zoning was disrupted during the course of the eruption due to the simultaneous withdrawal of magma from different portions of the reservoir. During the first phase of the eruption, high-silica tephriphonolitic to phonolitic melts, residing in the upper part of the reservoir, were emitted with less involvement of the deeper phonotephrite. Phonotephritic magmas, with only a minor contribution of evolved melts, were later erupted as spatter-rich pyroclastic flows that preceded the main caldera collapse. During the caldera collapse, the whole reservoir was involved, leading to extensive and intimate syn-eruption mingling of the different melts. A comparison between the Onano eruption and other eruptions of the Italian high potassic volcanism provides new insights into the evolution of the Roman Comagmatic Province magmas and their eruptive processes.