THE GEOCHEMISTRY OF BR IN MARINE EVAPORITE SYSTEMS

During the course of evaporation of seawater, halite is the first and most abundant Cl-mineral precipitated. Other relevant minerals are sylvite, kainite, carnallite, and bischofite. The primary or secondary origin of all these minerals in evaporite bodies can only be judged from trace element distribution. The conservative vertical distribution in the ocean, the high concentration of 0.84 mmol/kg H₂O and one of the highest residence times of 1.0 · 10⁸ - 7.9 · 10⁸ years in the oceans make Br the most important trace element in chemically precipitated marine Cl-sediments. In contrast to the high number of papers dealing with halite, investigations on other marine Cl-minerals are rare. Evaporation experiments with natural and synthetic seawater indicate that the partition coefficient of Br is both a function of the Br concentration itself and the chemical composition of the solution. Thermodynamical modeling were carried out demonstrating a high sensitivity of the partition coefficient of Br on changes of mayor ion concentrations. In first stage evaporating seawater variations in SO₄ concentration show the highest influence on Br incorporation into halite, followed by K, HCO₃ and Ca. The addition of these ions decreases the incorporation of Br in halite, and Mg distinctly has the lowest influence. A very high effect of Mg is valid only for high concentrations as in late stages of halite saturation in evaporating seawaters. The strong dependence of the partition coefficient on the chemical composition of the aqueous solution is of direct influence to the geochemical application of Br data. Every change in the composition of seawater, caused by dolomitization or secular changes in ocean ridge hydrothermal fluxes directly influence the incorporation of Br in halite during the evaporation and formation of marine sediments. Periods with low concentrations of MgSO₄ and high contents of K and Mg in ancient seawater should precipitate basal halite with lower Br concentrations that recent seawater. This is the reason why the cretaceous marine evaporites from the Khorat plateau have only about 40 µg Br/g basal halite. These and other Br contents of primary basal marine halites with model data give strong evidence for rapid and extensive changes in the composition of Phanerozoic seawater.