STEP-FILLING AND DEVELOPMENT OF A 3-LAYER 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 fea tures of the deposit was carried out to investigate the behaviour of t he magma chamber before the eruption. The collected data show three di stinct compositional groups separated by gaps. Single depositional uni ts contain glass shards formed contemporaneously. Although each of the se shards is homogeneous they display the same compositional variation s and gaps detected in pumice clasts. This feature is taken as an evid ence 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 mama was alkali- trachyte and highly homogeneous, likely a consequence of vigorous conv ection. The intermediate magma was trachyte with a slight and continuo us compositional variation, likely resulting from less intense convect ion. The lowermost magma was compositionally zoned from alkali-trachyt e to latite downward. This compositional zonation was most likely acqu ired during uprise from a deeper reservoir. The three magmas entered t he chamber sequentially from the uppermost to the lowermost. The latte r 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 t he temporal succession of input of magma batches. Furthermore they als o suggest that the ma,ama bodies did not mix although, at least the up permost two, coexisted inside the chamber for a time long;enough to al low internal homogeneization by convection.