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

Products of the Pomici di Base plinian eruption of Somma-Vesuvius consist o f pumice and scoria fall deposits overlain by lithic-rich phreatomagmatic d eposits. The plinian fall, which represents most of the magma volume involv ed in the eruption, ranges in composition from trachyte (SiO2 = 62.5 wt%) t o latite (SiO2 approximate to 58 wt%) in the lower one-third of the deposit , whereas the upper two-thirds of the total thickness consists of latitic s coriae with fairly uniform composition (SiO2 approximate to 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 equilibriu m with the host rock. Minerals not in equilibrium, both in trachytic and la titic rocks, consist of discrete crystals of sanidine and plagioclase wette d by trachytic glass and felsic aggregates with interstital trachytic glass . Trends of major and trace elements are consistent with crystal-liquid fra ctionation processes and rule out syn-eruptive mixing processes between lat itic and trachytic magmas. We suggest that discrete crystals and crystal ag gregates not in equilibrium with the host rock represent fragments of the c rystallising boundary layer at the upper walls of the magma chamber, which were wrenched and admired into the magma during the ascent. This process di versifies the mineral assemblage and increases the crystal content of the r ocks. We propose that diffusive crystallization processes operating at the wall of the chamber allowed the formation of a two-fold layered reservoir w ith a more mafic, homogeneous lower body and a more evolved, compositionall y graded upper body. Around one-quarter of crystals adhering to the upper p art of the magma chamber were admired into the magma during the eruption. T he 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).