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

Abstract

 The study proposes a model by which a thick succession of volcanic tuffs can be zeolitized by alteration of pyroclastic material in the presence of sufficient eruptive water and at temperatures close to water vapour condensation. In the case of phreatomagmatic products, the model simplifies interpretation of problematic deposits that exhibit pronounced vertical and lateral variation in lithification grade. A major feature of the model is that thick zeolitized tuffs can be formed during emplacement of pyroclastic products, in marked contrast to later alteration in an open hydrologic system. Geological, volcanological and mineralogical data for the Neapolitan Yellow Tuff, a widespread trachytic pyroclastic deposit outcropping around Campi Flegrei (Southern Italy), have been used to infer the physico-chemical conditions that determined mineral genesis. This tuff shows a reduction in lithification 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, inhibiting zeolite crystallization. During rapid deposition of the thick, wet succession thermal insulation allowed the persistence of elevated temperatures for a time sufficient for enhancement of hydration-dissolution processes in the volcanic glass. The highly reactive alkali-trachytic glass quickly buffered the acid pH of the system, favouring phillipsite crystallization followed by chabazite nucleation. The variable zeolite content reflects fluctuating emplacement conditions (e.g. changes in water content and temperature). Cooling of the upper and relatively thin distal deposits inhibited the zeolitization process, thereby preserving the primary unlithified deposit.