Genesis of zeolites in the Neapolitan Yellow Tuff: geological, volcanological and mineralogical evidence

The study proposes a model by which a thick succession of volcanic tuffs ca n be zeolitized by alteration of pyroclastic material in the presence of su fficient eruptive water and at temperatures close to water vapour condensat ion. In the case of phreatomagmatic products, the model simplifies interpre tation of problematic deposits that exhibit pronounced vertical and lateral variation in lithification grade. A major feature of the model is that thi ck zeolitized tuffs can be formed during emplacement of pyroclastic product s, in marked contrast to later alteration in an open hydrologic system. Geo logical, volcanological and mineralogical data for the Neapolitan Yellow Tu ff, a widespread trachytic pyroclastic deposit outcropping around Campi Fle grei (Southern Italy), have been used to infer the physicochemical conditio ns that determined mineral genesis. This tuff shows a reduction in lithific ation grade towards the base, top and with distance from the vent and very variable zeolitization within the lithified portion. We suggest that during initial emplacement the erupted products chilled against the ground, inhib iting zeolite crystallization. During rapid deposition of the thick, wet su ccession thermal insulation allowed the persistence of elevated temperature s for a time sufficient for enhancement of hydration-dissolution processes in the volcanic glass. The highly reactive alkali-trachytic glass quickly b uffered the acid pH of the system, favouring phillipsite crystallization fo llowed by chabazite nucleation. The variable zeolite content reflects fluct uating emplacement conditions (e.g. changes in water content and temperatur e). Cooling of the upper and relatively thin distal deposits inhibited the zeolitization process, thereby preserving the primary unlithified deposit.