Can we recognize ancient, undersaturated tuffs using zeolitic reaction paths and major and trace element behaviours?

Volcanic glasses that are alkaline and silica undersaturated have been extensively reported as precursor components of zeolitized tuffs, in the context of diagenetic reactionsa. Indeed, more recent studies on mineral stabilities and fluid constraints on zeolitic reactions point to low silica, high pH and high alkali activities as “ideal” conditions for zeolitization. Therefore, to recognize and better understand the preservation of silica undersaturated extrusives in the geological record, we need to study reaction paths of zeolite-group minerals, as well as those of clays and other associated reaction-product phases.Accessing geochemical signatures from these successions is inherently problematical due to (potentially multiple) phase transformations involved in the reactive sequences, and the concurrent element mobilizations associated with these changes. Here, we present research in progress aiming to examine trace element behaviours and initial signatures in the first-formed alteration minerals in very young tuffs from Italy and East Africa. Our approach is to adapt a sensitive, in situ, analytical technique (LA-ICPMS), currently used for analysis of glass in tephra, for this new application in paragenetically-constrained zeolitized tuffs. We report on preliminary results, and compare them with previous data determined by bulk cation exchange.Improved recognition of precursor compositions has potential REE exploration implications, through the association of alkaline rocks with carbonatite suitesb.aBish and Ming (2001) Rev.Min. Geochem., 45.bCampbell et al (2012) Min. Deposita, 47, 371-382.