Stable chlorine isotopes in arid non-marine basins: Instances and possible fractionation mechanisms

Abstract

Stable chlorine isotopes are useful geochemical tracers in processes involving the formation and evolution of evaporitic halite. Halite and dissolved chloride in groundwater that has interacted with halite in arid non-marine basins has a delta Cl-37 range of 0 +/- 3 parts per thousand, far greater than the range for marine evaporites. Basins characterized by high positive (-1 to +3 parts per thousand), near-0%, and negative (-0.3 to -2.6%) are documented. Halite in weathered crusts of sedimentary rocks has delta Cl-37 values as high as +5.6 parts per thousand. Salt-excluding halophyte plants excrete salt with a delta Cl-37 range of -2.1 to -0.8%. Differentiated rock chloride sources exist, e.g. in granitoid micas, but cannot provide sufficient chloride to account for the observed data. Single-pass application of known fractionating mechanisms, equilibrium salt-crystal interaction and disequilibrium diffusive transport, cannot account for the large ranges of delta Cl-37. Cumulative fractionation as a result of multiple wetting-drying cycles in vadose playas that produce halite crusts can produce observed positive delta Cl-37 values in hundreds to thousands of cycles. Diffusive isotope fractionation as a result of multiple wetting-drying cycles operating at a spatial scale of 1-10 cm can produce high delta Cl-37 values in residual halite. Chloride in rainwater is subject to complex fractionation, but develops negative delta Cl-37 values in certain situations; such may explain halite deposits with bulk negative delta Cl-37 values. Future field studies will benefit from a better understanding of hydrology and rainwater chemistry, and systematic collection of data for both Cl and Br.