REGIONAL DOLOMITIZATION: THE LEAKY-AQUIFER ASCENDING BRINE MODEL

A conceptual model for regional dolomitization of limestone is proposed in which leaky aquifer systems permit upward cross-formational transport of deeper formation brines. Mixing of lower TDS intraformational water with ascending brine, combined with decreasing temperature along the geothermal gradient traversed by the brines creates a pore fluid that is thermodynamically undersaturated with respect to calcite and supersaturated with respect to dolomite and gypsum. The conceptual model was tested on the regionally extensive Miocene age Gachsaran Fm in the United Arab Emirates, which has been tectonically uplifted and extensively dolomitized. Uranium isotopic (²³⁴U/²³⁸U) activity ratio disequilibrium suggests that the formation has recently (<100,000 y) been a chemically open system. This alteration most likely is the result of a massive conversion of previously existing normal marine Mg-calcite and aragonite to dolomite, and thus provides an unprecedented opportunity to study an area where active dolomitization has occurred. Measurement of solute concentration, temperature, pH, and hydrogeologic boundary conditions can be made, rather than relying on geologic proxies or assumptions about a long-extinct “geologic” system to try to understand dolomitization, as is now necessary. Solute mass balance, solid and liquid phase isotopes of strontium, sulfur, oxygen, and carbon, and solute transport modeling significantly constrain, and are consistent with, the leaky-aquifer ascending brine thermodynamic conceptual model. Because similar hydrogeologic conditions are common, this model expands substantially the range of potential dolomitization environments and, thus, provides insight into the ubiquitous presence and distribution of dolomite in the geologic record.