EXPERIMENTAL EXAMINATION OF THE MG-SILICATE SYSTEM AT AMBIENT TEMPERATURE: IMPLICATIONS FOR ALKALINE CHEMICAL SEDIMENTATION AND LACUSTRINE CARBONATE FORMATION

Despite their economic significance, Cretaceous presalt carbonates of the South Atlantic continental margins are not well-described by existing facies models, in part because of our poor understanding of the chemical processes that generate the distinctive sedimentary products characteristic of alkaline, non-marine environments. Here, we present the results of experiments designed to quantify the processes of alkaline chemical sedimentation. Using real-time observations of fluid chemistry, post-experiment analysis of precipitated solids, and modeling tools, we illustrate that spherulitic carbonates and Mg-silicate clays observed in presalt carbonates were likely precipitated from elevated pH (10-10.5) waters with high concentrations of silica and alkali cations typical of intermediate to felsic rocks. Charge balance constraints require that these cations were not counterbalanced to any significant degree by anions typical of seawater. Experimental data suggest that, at this alkaline pH, only modest Ca concentrations would have been required to precipitate spheroidal calcium carbonate. Given the rapid rates of CaCO₃ nucleation and growth under such conditions, it is unlikely that Ca concentrations ever exceeded these values, and extensive sediment accumulation therefore required sustained chemical fluxes. Moreover, our experiments indicate that the original mineralogy of presalt CaCO₃ could have been calcite or aragonite, but the differing time scales of precipitation between CaCO₃ and Mg-silicates would have tended to skew the Mg/Ca ratio in solution towards elevated values favoring aragonite. Mg-silicate nucleation and growth rates measured during our experiments suggest that elevated silica and high pH would have limited the Mg concentrations required to precipitate poorly crystalline Mg-silicates, which, through time, crystallize to minerals such as sepiolite and stevensite. Although our results provide robust constraints on the geochemistry of Mg-silicate-carbonate interactions during alkaline lake sedimentation, they leave open the potential for biological contributions to sedimentation within the presalt basins, as well as the hydrogeochemical mechanisms that maintained a productive carbonate factory of the scale observed along the South Atlantic margins.