Gypsum-anhydrite transformations in Messinian evaporites of central Tuscany (Italy)

The Messinian succession of Tuscany (central Italy) contains three evaporit ic units. Among the several exposed evaporitic lithofacies, only selenitic gypsum precipitated directly from evaporating brines. All the other facies, nodular microcrystalline gypsum, gypsarenites and gypsum laminites, despit e their macroscopic differences, display the same petrographic textures, in dicating that they are the product of dehydration of gypsum to give anhydri te which has been successively rehydrated to secondary gypsum. These second ary facies show an entire array of textures ranging from cloudy ameboid (xe notopic) with anhydrite relies, to idiotopic without anhydrite relies, that are here interpreted as a sequence of progressive stages of rehydration. T he presence of completely hydrated petrofacies at the core of nodules which display a less hydrated rim suggests that these rocks have undergone at le ast two cycles of a dehydration-rehydration process. This interpretation is supported by the presence of satin spar veins that are replaced by microcr ystalline gypsum. Satin spar itself is considered to be a by-product of anh ydrite hydration. The first dehydration-rehydration event affected the enti re gypsum deposit, producing a completely hydrated (idiotopic) facies toget her with satin spar veins; the second affected only veins, fractures and th e rims of nodules, turning the first generation of satin spar and idiotopic gypsum into cloudy ameboid gypsum, Sedimentary structures typical of sabkh a environments indicate for the youngest formation that the first dehydrati on and rehydration process occurred syndepositionally. The preservation of primary gypsum facies only at sites with condensed sections, indicate for t he oldest two formations that the first dehydration event occurred upon bur ial, This event has been estimated to have occurred in the earliest Pliocen e. After the Early Pliocene, dehydration was favored even at shallow depths , due to an increased heat flow related with the emplacement of local crust al magmatic bodies. Rehydration possibly occurred when these formations wer e uplifted and exposed to ground and/or meteoric water, The Volterra Basin has undergone alternating subsidence and uplift events, that can account fo r two dehydration-rehydration processes at least, also driven by alternatin g circulation, in the tectonic fractures, of fresh and salty water, the lat ter derived from dissolution of Messinian halite. (C) 2000 Elsevier Science B.V. All rights reserved.