Chemical zoning and crystallization mechanisms in the magma chamber of the Pomici di Base plinian eruption of Somma-Vesuvius (Italy)

Abstract

Products of the Pomici di Base plinian eruption of Somma-Vesuvius consist of pumice and scoria fall deposits overlain by lithic-rich phreatomagmatic deposits. The plinian fall, which represents most of the magma volume involved in the eruption, ranges in composition from trachyte (SiO₂ = 62.5 wt%) to latite (SiO₂ ≈ 58 wt%) in the lower one-third of the deposit, whereas the upper two-thirds of the total thickness consists of latitic scoriae with fairly uniform composition (SiO₂ ≈ 55–56 wt%). All the products have very low content of phenocrysts (from 4 wt% in trachyte pumice to 1 wt% in the latite scoriae), most of which are not in equilibrium with the host rock. Minerals not in equilibrium, both in trachytic and latitic rocks, consist of discrete crystals of sanidine and plagioclase wetted by trachytic glass and felsic aggregates with interstital trachytic glass. Trends of major and trace elements are consistent with crystal-liquid fractionation processes and rule out syn-eruptive mixing processes between latitic and trachytic magmas. We suggest that discrete crystals and crystal aggregates not in equilibrium with the host rock represent fragments of the crystallising boundary layer at the upper walls of the magma chamber, which were wrenched and admixed into the magma during the ascent. This process diversifies the mineral assemblage and increases the crystal content of the rocks. We propose that diffusive crystallization processes operating at the wall of the chamber allowed the formation of a two-fold layered reservoir with a more mafic, homogeneous lower body and a more evolved, compositionally graded upper body. Around one-quarter of crystals adhering to the upper part of the magma chamber were admixed into the magma during the eruption. The absence of significant syn-eruptive mixing processes and the major role played by diffusive crystallization are consistent with a low aspect ratio magma chamber (width/height <1).