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

Abstract

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 (AD 1538). However, there have been more energetic Plinian events, i.e., the Agnano Monte Spina eruption (4.1 ky), and very highly-explosive, caldera-forming eruptions, i.e., the Campanian Ignimbrite eruption (39 ky). 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 physico-chemical 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 suggest that this parameter may be influenced by magma/water interaction and/or magma chamber geometry and replenishment.