OXYGEN ISOTOPIC EXCHANGE DURING DIAGENETIC RECRYSTALLIZATION IN THE CALCARE-DI-BASE MEMBER OF THE GESSOSO-SOLFIFERA FORMATION, SICILY

An isotopic exchange model between dolomite-calcite pairs and circulat ing fluids has been applied to the ''Calcare di Base'' member of the M essinian evaporitic sequences of Sicily (Upper Miocene). SEM observati ons have been also performed. The observed calcite-dolomite deltaO-18 values are in good agreement with the respective compositional and tex tural features. In case of open system conditions, blocky spar fills t he porosity of the microcrystalline dolomite matrix and strontium mine rals are absent. These are evidences of diffuse secondary crystallizat ion under high water/rock ratio. Closed system conditions are characte rized by the occurrence of secondary calcite-celestite associations in the pore spaces. In this case only moderate and localized water-rock interaction took place and the isotopic imprint of the original paleoe nvironment has been still maintained.