Step-filling and development of a three-layers magma chamber: The Neapolitan Yellow Tuff case history

The Neapolitan Yellow Tuff, the product of the largest known trachytic phreatoplinian eruption, gives a good opportunity to investigate the filling mechanisms and internal dynamics of a trachytic magma chamber. A detailed study of the geochemical, mineralogical and isotopical features of the deposit was carried out to investigate the behaviour of the magma chamber before the eruption. The collected data show three distinct compositional groups separated by gaps. Single depositional units contain glass shards formed contemporaneously. Although each of these shards is homogeneous they display the same compositional variations and gaps detected in pumice clasts. This feature is taken as an evidence for interpreting the detected compositional gaps as real gaps in the chamber. Therefore the chamber was filled by three distinct magma bodies separated by compositional gaps. The uppermost magma was alkali-trachyte and highly homogeneous, likely a consequence of vigorous convection. The intermediate magma was trachyte with a slight and continuous compositional variation, likely resulting from less intense convection. The lowermost magma was compositionally zoned from alkali-trachyte to latite downward. This compositional zonation was most likely acquired during uprise from a deeper reservoir. The three magmas entered the chamber sequentially from the uppermost to the lowermost. The latter entered the chamber short before the beginning of the eruption. Its input was interpreted as a possible triggering factor for the eruption. The results of this study strongly support a step-filling mechanism for the Neapolitan Yellow Tuff magma chamber and allow definition of the temporal succession of input of magma batches. Furthermore they also suggest that the magma bodies did not mix although, at least the uppermost two, coexisted inside the chamber for a time long enough to allow internal homogeneization by convection.