Depositional mechanisms and alteration processes in different types of pyroclastic deposits from Campi Flegrei volcanic field (Southern Italy)

The Campi Flegrei volcanic field, Southern Italy, is composed of a large variety of pyroclastic deposits related to both magmatic and hydromagmatic activity, In spite of similarities of composition, grain size, age and pre-eruptive environment, the various deposits exhibit great differences in terms of degree and type of alteration of the juvenile components. In particular, detailed analyses of pyroclasts collected in the numerous monogenetic volcanic cones show the broad range of alteration, from fresh glass to complete replacement of the glass by the zeolites. The comparative analysis of 17 different deposits has allowed us to conclude that the eruptive and emplacement conditions were the major controls on the zeolitization processes. The relationships between voIcanological features of deposits and secondary mineralization are consistent with the glass-fluid phase reaction in a near-closed post-depositional system. Eruptions characterised by separation of water vapour from the pyroclastic particles produce unaltered or poorly altered deposits. In contrast, eruptions characterised by mechanisms that permit the capture and retention of pore water and water vapour produce zeolitized deposits. According to these lines of evidence the zeolitization of pyroclastic products of Campi Flegrei took place immediately after emplacement, during the cooling of wet deposits. (C) 1999 Elsevier Science B.V. All rights reserved.