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Ascending Freshwater-Mesohaline Mixing Dolomitization: Insights from Geochemical and Reactive Transport Models

Abstract

La Molata, southeast Spain, is a Miocene outcrop which has been extensively dolomitized. The fluid inclusion and isotopic data indicate that this dolomitization occurred from ascending freshwater-mesohaline mixing. The conceptual model suggests CO2-rich freshwater flows from the underlying fractured volcanic basement into the carbonate succession and, through vertical flow and mixing with the mesohaline seawater, generates extensive dolomitization. Controls on this conceptual model were evaluated using geochemical and reactive transport modeling. Geochemical mixing calculations have shown that, for a wide range of freshwater and mesohaline seawater end-member compositions, at surface temperatures and pressures, nonlinear mixing behavior generates the so-called “Dorag zone” where it is thermodynamically favorable to dissolve calcite and precipitate dolomite. In order to evaluate kinetic and hydrodynamic controls, 2D reactive transport models were generated representing two freshwater and seawater mixing scenarios: traditional coastal mixing zone and ground water recharge into the base of the carbonates causing ascending mixing. Some key geological variables, such as freshwater and seawater compositions, temperature, PCO2 of the freshwater, and ground water flow velocity were tested for their control and sensitivity. Although the geochemical modeling confirmed the dolomitization potential of mixing, the reactive transport simulations failed to generate extensive dolomite in the traditional coastal mixing zone, within 1 million years. However, initial simulations with ascending freshwater-mesohaline mixing suggest that rapid dolomitization can occur under certain hydrological conditions. Additional work is being undertaken to further constrain the complex regional flow regime using a basin simulation model. The reactive transport simulations suggest that early dolomitization by mixed waters requires an additional driver to overcome the kinetic barrier, such as higher temperature fluids and/or other kinetic drives. Although mixing within a salinity stratified coastal aquifer may not be a feasible mechanism for formation of extensive replacement dolomite, vertical fluid flow and circulation of reactive fluids in an ascending mixing setting has considerably greater potential.