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Cicerali, D.; Arslan, M.; Yazar, E.A.; Yücel, C.; Temizel, İ.; Park, S.-H. (1. November 2019): Mineralogy, chemistry, and genesis of zeolitization in Eocene tuffs from the Bayburt area (NE Turkey). Constraints on alteration processes of acidic pyroclastic deposits. In: Journal of African Earth Sciences, Vol. 162, 103690: pp. 1-14
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Abstract

Eocene vitric tuffs in the Bayburt area (NE Turkey) possess a dacite-rhyodacite composition and contain abundant altered glass shards and flattened fibrous pumice with lesser amounts of pyrogenic minerals (plagi-oclase, quartz, biotite, and sanidine). Zeolitization is common and generally develops in the glassy components. The main zeolite mineral found in the tuffs is clinoptilolite, with lesser amounts of analcime and mordenite (?). Smectite is the dominant clay mineral, with lesser amounts of kaolinite also present. Opal-CT, calcite, and dolomite occur as secondary minerals. Scanning electron microscopy studies reveal the dissolution of glass, with the development of popcorn-textured smectites and tabular clinoptilolites developed. The opal-CT spheres found on the clinoptilolite crystals appear to have crystallized simultaneously and are due to excess silica in the en-vironment. Fibrous mordenite (?) also developed on the clinoptilolites. In the clinoptilolites Rr = 0.81–0.82, Si/Al = 4.20–4.70, and K+ is the dominant exchangeable cation (0.65–3.30) seen, together with lesser amount of Ca2+ (0.39–2.25) and Na+ (0.02–2.16). The degree of zeolitization increases with the content of glassy com-ponent in the tuffs. The formation of K-clinoptilolite is controlled by the SiO2/Al2O3 (4.9–7.0) and K2O/Na2O (0.8–10.8) ratios in the zeolitized tuffs. δ18OV-SMOW and δDV-SMOW values of clinoptilolite-rich tuffs range from 22 to 25.6‰ and from −96 to −111‰, respectively, which suggests that zeolitization occurred at low tem-peratures (30-55 °C) via fluid-rock interactions. Smectite was initially formed from the devitrification of glassy components and is associated with the increase in (Na++K++Ca2+)/H+ activity and the presence of high pH (~9–10) fluids. The zeolites subsequently developed within a saline-alkali shallow marine environment.