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 northernmostsector of the Roman Comagmatic Province (Latium, Italy). The stratigraphic sequence, from base to top, includes ash andpumice-rich flow deposits, spatter-rich flow deposits and lag breccias associated with ash-rich flow deposits. By combiningmajor and trace element compositions of the bulk rocks, matrix glasses and minerals of juvenile components from thedifferent depositional units, we reconstruct the pre-eruptive evolution of the magma chamber and the syn-eruptive magmadynamics. Juvenile clasts with heterogeneous glass composition and/or mineral assemblage are a ubiquitous feature of theOnano 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 theerupted melts. The mafic mineral assemblage suggests that the pre-eruption magma reservoir was periodically perturbed bythe arrival of a primitive magma carrying on Mg-rich minerals and/or remobilizing a mafic crystal mush at the bottom ofthe reservoir. According to the results obtained from both rhyolite-MELTS and Rayleigh crystal fractionation modeling, weinfer that the system evolved from phonotephrite to phonolite both via crystal fractionation and magma mixing between thetwo end members. Crystallization mostly proceeded at the wall of the reservoir, while magma mixing in the middle zonesof the chamber generated the intermediate tephriphonolitic melts. The pre-eruptive chemical zoning was disrupted duringthe course of the eruption due to the simultaneous withdrawal of magma from different portions of the reservoir. Duringthe 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 contributionof evolved melts, were later erupted as spatter-rich pyroclastic flows that preceded the main caldera collapse. During thecaldera collapse, the whole reservoir was involved, leading to extensive and intimate syn-eruption mingling of the differentmelts. A comparison between the Onano eruption and other eruptions of the Italian high potassic volcanism provides newinsights into the evolution of the Roman Comagmatic Province magmas and their eruptive processes.