Dolomitizing Seas in Evaporitic Basins: A Model for Pervasive Dolomitization of Upper Miocene Reefal Carbonates in the Western Mediterranean

Abstract

Petrographic, stratigraphic, and stable-isotopic data from a dolomitized upper Miocene reef complex on Mallorca, Spain, coupled with the presence of extensive Messinian subtidal evaporite deposits throughout the western Mediterranean suggests that Messinian evaporitic brines were responsible for dolomitization. Thermodynamic reaction-path modeling was undertaken to more rigorously and quantitatively demonstrate the dolomitizing potential of this evaporitic system. These calculations provide maximum amounts of dissolution or precipitation (moles/L) to maintain a specified fluid-rock equilibrium. Significant amounts of dolomite can be created when brines ar concentrated by evaporation prior to reaction with calcite. Heavy stable isotopes of Mallorcan dolomites are consistent with an evaporitic origin; however, lighter stable isotopes do not preclude this model. Calculations show that mixing evolved brines with unaltered seawater or continental waters produces a suite of fluids with much greater dolomitization potential than seawater and oxygen-isotope signatures approaching those of seawater. This evaporitic-basin model serves as one hypothesis for the many dolomitized upper Miocene reef complexes around the western Mediterranean, and may be applicable to many older Phanerozoic carbonates associated with basinal evaporites.