ON THE SECULAR VARIATIONS IN THE COMPOSITION OF PHANEROZOIC MARINE POTASH EVAPORITES

In a recent paper, L. A. Hardie proposed that the secular changes in t he mineralogy of marine nonskeletal limestones and in the mineralogy o f marine potash evaporites during the Phanerozoic are the result of ch anges in the composition of seawater caused primarily by fluctuations in the flux of seawater through mid-ocean ridges, He suggested that ev en quite small variations (25%) in this flux have a profound effect on the composition of seawater, We show that the effects of changes in t he hydrothermal flux are much smaller than he proposed. Nevertheless, a doubling of the hydrothermal flux could probably alter the compositi on of seawater sufficiently to affect the mineralogy of marine potash evaporites. An alternative explanation for the variations in the miner alogy of these deposits invokes differences in the degree of dolomitiz ation during the evaporation of seawater. The apparent near constancy of the K+ content of seawater during the Phanerozoic supports this hyp othesis, It seems likely that during periods of rapid seafloor spreadi ng the rate of seawater cycling is greater than today, that sea level is higher, that very large carbonate platforms are more abundant, and that changes in seawater chemistry together with increases in dolomiti zation during the evaporation of seawater lead to MgSO4-free potash de posits, The relative importance of these several effects can probably be determined with certainty only when we have analyses of relatively unaltered seawater from the several Phanerozoic periods.