Is Cl isotope fractionation in the seawater-evaporite system dependent on seawater chemistry?

Abstract

Published values, near 1.0003, of the fractionation factor for stable Cl isotopes between halite and brine (αhal-br) fail to account for several sets of δ37Cl data for marine evaporitic halite. Clustered negative values of δ37Cl data, including some that appear to correspond to basal halite, would require measurable secular change in δ37Cl of marine chloride during the Phanerozoic if αhal-br is invariant. Such change is inconsistent with understanding of the geochemical cycle of Cl. Alternatively, marine δ37Cl has remained constant, but αhal-br has undergone secular change between values of about 1.0003 and 0.9996 in Phanerozoic marine evaporite settings. Such an interpretation is favored by the occurrence of clustered negative δ37Cl values at times when potash-facies evaporite contained sylvite rather than MgSO4 minerals, reflecting long-term secular change in seawater chemistry. Values of αhal-br may therefore depend on brine composition.