Genesis of zeolites in the Neapolitan Yellow Tuff: geological, volcanological and mineralogical evidence |
| |
| Authors: | Maurizio de'Gennaro Piergiulio Cappelletti Alessio Langella Annamaria Perrotta Claudio Scarpati |
| |
| Institution: | (1) Dipartimento di Scienze della Terra, Università FEDERICO II, Via Mezzocannone 8, I-80134 Napoli, Italy Tel.: +39-081-5473363; Fax: +39-081-5518701; E-mail: degennar@unina.it, IT;(2) Facoltà di Scienze, Università del Sannio, Via Port'Arsa 11, I-72100 Benevento, Italy, IT;(3) Dipartimento di Geofisica e Vulcanologia, Università FEDERICO II, Largo S. Marcellino 10, I-80138 Napoli, Italy, IT |
| |
| 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.
Received: 25 May 1999 / Accepted: 28 October 1999 |
| |
| Keywords: | |
| 本文献已被 SpringerLink 等数据库收录！ |
|
