Explosive mafic eruptions: the fall-out phase of the "Pozzolane Nere Fm." (Colli Albani Volcanic District, Rome)

Abstract

Colli Albani (Roma) is a composite quaternary volcano that became active at approximately 600 ka and that is now considered to be in a quiescent state. Both explosive and effusive products are remarkably undersaturated and belong to the HK-Series. In spite of its very silica poor compositions, the volcano displays features identical to those of felsic volcanoes, like low aspect ratio ignimbrites, tens of cubic kilometres large, and a related central (8x8 km) collapse caldera. Despite the widespread literature about the structure and history of the volcano, the mechanisms governing the explosive activity of these very undersaturared magmas are still unknown. In order to improve our understanding on the origin of these high-energy eruption, we carried out a detailed investigation of one of the largest explosive events in the history of the volcano (407 ka, Vulcano Laziale phase), the Pozzolane nere formation (PNR).. The PNR formation is characterized by a basal scoria fall deposit showing an east-trending axis of dispersion and a maximum thickness of about 2 meters overlain by a widespread low aspect ratio ignimbrite, estimated at 30km3 as bulk volume. In this study, we analyse in detail the textural characteristics and physical properties of the scoriaceous PNR basal fall-out deposit. For our investigation, we have chosen two different sites: i) Artena, i.e. a proximal deposit situated along the maximum axis of dispersion and ii) Tivoli, i.e. a distal deposit situated north-east with respect to the eruption centre. In each level, 50 scoria clasts have been selected to determine density and vesicularity by He picnometer. Petrological and minero-chemical analyses of the samples were performed by a combination of SEM and microprobe techniques. We analysed the internal textures and structure of the scoria clasts in order to assess the role of vesicles and microlites in eruptive style and history of the eruption. Results from the vesicle size distribution of the selected scoriae suggest an uneven distribution of nucleation events. Various clasts types have been recognized with different history of nucleation and growth processes. Bubble Number Densities (BNDs) vary from 10^6 at the bottom of the deposit, similar to those known for violent strombolian eruptions, to 10^5 in the middle, returning to 10^6 at the top, and up to10^7 in the overlying ignimbrite, indicating that the eruption was increasing in energy. Crystal fraction of the scoriae sample is about phi=0.10 with leucite microphenocrysts (>50 microns) and leucite + clinopyroxenes "microlites" (<20 microns). Textural and minero-chemical investigations of the samples have been combined with their rheological characterization, in order to model the magma dynamics leading to the high-energy eruption of PNR. Pure liquid anhydrous viscosity of the remelted PNR fall-out at high (1124°C<T<1569°C) and low T (690°C<T<800°C) was performed by a combination of concentric cylinder measurements and micropenetration technique. Multiphase rheology was computed through the Vona et al. model for crystal bearing magmas both for the fall out and the PNR flow.