SECULAR VARIATION IN SEAWATER CHEMISTRY DURING THE PHANEROZOIC AS INDICATED BY BRINE INCLUSIONS IN HALITE

Analysis of data on the chemical composition of primary fluid inclusio ns (brine inclusions) in primary-bedded halite from many evaporite for mations of northern Pangea shows that during the Phanerozoic the chemi cal composition of marine brines was oscillating significantly between the Na-K-Mg-Ca-Cl type and the Na-K-Mg-Cl-SO4 type. We regard those c hanges as corresponding to the chemical evolution of the Phanerozoic o cean. The changes correlate in time with earlier suggested secular cha nges in the mineralogies of marine nonskeletal limestones and potash e vaporites. In addition to those secular changes of seawater chemistry, the concentrations of K, Mg, Ca, and SO4 ions in marine brines underw ent important changes under the influence of local factors, including rock-water interaction. These secondary changes did not influence the principal chemical ty-pe of the brine characteristic for a given time interval. In the past concentration of the Ca-ion did not exceed the p resent concentration in marine water by a factor of three, and the inc rease was synchronous with a decrease in the SO4-ion concentration. Th is could be as much as three times lower when compared to the present concentration of that ion in seawater.