Dolomitizing Seas in Evaporitic Basins: A Model for Pervasive Dolomitization of Upper Miocene Reefal Carbonates in the Western Mediterranean
OSWALD, E. J., M. A. A. SCHOONEN, and W. J. MEYERS, SUNY at Stony Brook, Stony Brook, NY
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 (PHRQPITZ, Plummer et al., 1988) 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 are concentrated by evaporation prior to reaction with calcite. For our scenario, two suites of seawater-brine compositions were calculated for progressive evaporation to 70X seawater. Brines were open to CO(2), and were allowed to precipitate dolomite, gypsum (3.6X), and halite (10.6X) upon saturation. The latter suite was finally allowed to equilibrate with (dissolve) calcite, precipitating dolomite. The calculated compositions of the brines not reacted with calcite agree well with data from modern marine evaporitic pans. Although seawater and its derivative evaporitic brines have a thermodynamic potential to precipitate dolomite, the amounts of dolomite formed are vanishingly small [<10(-3) moles/L] unless CO(3)= can be annibalized from preexisting carbonates. For brines allowed to back-react with calcite, significantly greater amounts of dolomite are formed. For example, at 40X seawater a liter of brine yields less than 10(-5) moles of dolomite, 0.038 moles of gypsum, and 1.67 moles of halite. Allowing this brine to equilibrate with calcite yields five orders of magnitude more dolomite (1.84 moles) and greater amounts of gypsum (0.77 moles), while dissolving 3.68 moles of calcite and 0.71 moles of halite. The thermodynamic/mass-balance potential (moles dolomite/L) of these brines is far greater than normal seawater or freshwater-seawater mixtures, hence the only pump needed may be passive marine circulation through the reefs. Large (50+ m) relative sea-level oscillations could bring evaporitic basinal rines in and out of contact with calcitic platforms. Thus brines could evolve out of contact with calcite during more restricted lowstands, and regionally dolomitize carbonates during highstands.
Heavy stable isotopes of Mallorcan dolomites [O(18) = +4.5 to +6.3%, C(13) = -0.1 to +3.14% PDB) 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.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)