Sulfur isotopic variations during seawater evaporation with fractional crystallization

Seawater was evaporated, stepwise isothermally at 23.5°C, for 73 days, up to a degree of evaporation of 138 × by H₂O weight. At various stages of evaporation the precipitate was totally removed from the brine and the latter was allowed to evaporate further. The sulfur isotopic compositions of the precipitates and related brines show the following characteristics: The initial δ²⁴S of the original seawater is +20‰. The δ³⁴S of both precipitates and associated brines decrease gradually in the gypsum field up to the end of the halite field, where δ³⁴S_precipitate = + 19.09‰andδ³⁴S_brine = + 18.40‰. The precipitates are always enriched in ³⁴S relative to the associated brines in these fields, but the enrichment becomes smaller towards the end of the halite field. A crossover. where the δ³⁴S of the brines becomes higher than those of the precipitates, occurs at the beginning of the Mg-sulfate field. The δ³⁴S_precipitate increases from + 19.09‰ at the end of the halite field through +19.35‰ in the Mg-sulfate field to + 19.85‰ in the K-Mg-sulfate field, whereas the δ³⁴S_brine increased from +18.40‰, through +20.91‰ to +20.94‰, respectively. This evolution implies different values of fractionation factors (α) for the minerals precipitated at the late halite, Mg-sulfate and K-Mg-sulfate fields, other than that for gypsum (1.00165). The value of α_{precipitate-residual brine} would then be very slightly >1 in the late halite field and <1 in the two later fields.

The experimental pattern of evolution of the δ³⁴S-values of the precipitates is in good agreement with data for natural anhydrites interbedded in halites, where δ³⁴S-values are lower relative to basal gypsum (and secondary anhydrite), and of primary minerals of the Mg- and K-Mg-sulfate facies, reported in evaporitic sequences, such as those of the Delaware (U.S.A.) and of the Zechstein (Germany) basins. Thus, these results shed new light on observations of natural evaporitic sequences and suggest that the compositional trend of δ³⁴S under the present experimental conditions may simulate and explain natural evaporitic processes.