Calcium Isotope Fractionation in Ca-Bearing Phases of Marine Evaporites

Calcium isotope fractionation recorded in calcium bearing phases (gypsum, anhydrite) of marine evaporites may give useful clues to the changing chemistry of concentrating brines through evaporative cycles. In order to understand the behavior of calcium isotopes within a brine, seawater evaporation experiments have been preformed to ascertain the fractionation factor of calcium isotopes (d44Ca value) with regard to precipitated anhydrite as the brine concentrates through the anhydrite saturated zone of evaporating seawater. These experiments show that the first anhydrite crystals precipitated from modern seawater at 40° C have a d44Ca value of -1.025 +/- 0.20 per mil, while subsequent generations of crystals have progressively heavier values up to -0.27 per mil, at which point halite begins to precipitate and Ca-sulfate precipitation decreases dramatically. Fractionation follows a predicted Rayleigh Fractionation pattern if a -1.0 per mil initial fractionation factor is used. Application of these Calcium isotope fractionation experiments to ancient marine evaporites, such as the Mediterranean evaporite deposits of the Messinian, could track oscillating brine chemistry through calcium dominated massive gypsum and anhydrite deposits. In this case, the isolation of the Mediterranean basin during the Messinian from the influence of Atlantic waters presents a unique case study for the effect of brackish and riverine waters on calcium isotopes in a closed ocean basin. In theory, the calcium isotopes of the evaporites should reflect the composition of the waters in which they precipitated and will indicate the relative importance of fresh or saline waters into the basin throughout evaporitic cycles in the Messinian.