Texture and composition of pumices and scoriae from the Campi Flegrei caldera (Italy): Implications on the dynamics of explosive eruptions

Abstract

[1] The Campi Flegrei (CF) caldera is one of the most dangerous quiescent volcanic systems in the world. Its activity mostly resulted in low-magnitude explosive eruptions, such as that of the Monte Nuovo tuff cone that represents the last eruptive event within the caldera (A.D. 1538). However, there have been more energetic Plinian events, e.g., the Agnano Monte Spina eruption (4.1 ka), and very highly explosive, caldera-forming eruptions, e.g., the Campanian Ignimbrite eruption (39 ka). Here, we integrate new and literature data on the groundmass texture and composition of pyroclastic products from the three above eruptions with the aim of unraveling how volatiles content, degassing mechanisms, and crystallization processes influence magma explosivity and eruption dynamics at CF. Previous studies indicate that the investigated rocks share similar major element bulk and phenocryst chemistry; also similar is the water content of their trapped melt inclusions. These observations suggest that the magmas feeding these eruptions had comparable physicochemical properties during storage in the shallow crust. However, our investigations indicate that the studied rocks differ in texture and composition of the groundmass and viscosity of the related magmas. We ascribe such differences to the variable style of volatile exsolution and outgassing from the melt, primarily in response to changes of the rate of magma ascent to the surface. We conclude that the magma ascent rate was the key parameter in driving explosive eruptions at CF, and we G 3

Permeability measurements of Campi Flegrei pyroclastic products: An example from the Campanian Ignimbrite and Monte Nuovo eruptions

“…5). In fact, although at the same water content the melt viscosity of KPT rhyolites is higher than that of MTN and CI trachytic melts, comparable values of calculated viscosity (logη (Pa s) ≃ 4 +/− 1; as these are calculations and not measurements they have an uncertainty of the order of 1 log unit) are obtained by assuming a dissolved water content of 7 wt.% for the KPT magma (Bachmann, 2010) (logη (Pa s) ≃ 5), 3 wt.% for the MTN magma (Piochi et al, 2008) (logη (Pa s) ≃ 5), 4 wt.% for the CI magma (Piochi et al, 2008) (logη (Pa s) ≃ 3.5), and very low water content for the Ambrym magma (Allard et al, 2009) (logη (Pa s) ≃ 3). In this view, we suggest that the high permeabilities of the MTN samples could be generated by a combination of high magnetite microlite content (Fig.…”

“…Second, CI is the largest magnitude explosive event of the Mediterranean region in late Quaternary (dating 39 ka, De Vivo et al, 2001), while MTN is the last eruption that occurred in the caldera in A.D. 1538 (Di Vito et al, 1987). Finally, there exists in the literature a lot of information on these eruptions about their stratigraphy, deposit characteristics, eruption dynamics, compositional, geochemical and textural data that allows us to interpret the results of this study in a robust scientific context (Rosi et al, 1996;Civetta et al, 1997;Rosi et al, 1999;Polacci et al, 2003;D'Oriano et al, 2005;Piochi et al, 2005;Pappalardo et al, 2007;Piochi et al, 2008, and references therein).…”

“…Large, irregularly-shaped, interconnected microcrack-like vesicles are very common in the scoria samples ( Fig. 2c) (Piochi et al, 2008).…”

“…2b). The MTN scoria and pumice clasts are characterized by a microlite-bearing (crystal content between 25-32 vol%, D'Oriano et al, 2005;Piochi et al, 2008) groundmass with tabular, acicular, and dendritic alkali-feldpar microlites, as well as widespread magnetite microlites in the microlite-richer scoriae (Fig. 2c).…”

“…Here we report the first permeability measurements performed on trachy-phonolitic pyroclastic products from the Campanian Ignimbrite (CI) and Monte Nuovo (MTN) eruptions, two explosive eruptions from CF that have very similar compositions but strongly differ in intensity, magnitude and eruption dynamics (Piochi et al, 2008 and references therein). The obtained results significantly enlarge the existing limited database on the permeability of CF magmas providing an important contribution to understanding and modelling degassing and eruptive processes at this highly risky volcanic caldera.…”