Additional Information Provided from the Analyses of Stable Chlorine and Bromine Isotopes in Formation Fluids from Sedimentary Basins

Literature concerning sedimentary basinal fluids usually compares the dominant halide ions chloride and bromide. Their evolution during seawater evaporation and mineral formation has been well documented. Recently, isotopic researchers have worked to expand the usefulness of chlorine and bromine stable isotope analyses towards understanding basinal processes.

Formation waters from a number of sedimentary settings show δ³⁷Cl and δ⁸¹Br values are characterized by variations in the order of -1.3‰ to +1.82‰ for chlorine (SMOC) and -1.50‰ to +3.35‰ for bromine (SMOB). Interestingly, a positive trend was found, where enrichment of δ⁸¹Br is coupled to δ³⁷Cl enrichment.

The ability to use these two isotopic tools parallel to traditional chemistry, elemental ratio trends (e.g. Br/Cl) and isotopic parameters such as ¹⁸O, ²H and ⁸⁷Sr has revealed a number of hitherto unknown trends in several well studied basins. In the Paleozoic sequences in southern Ontario and Michigan, the δ³⁷Cl and δ⁸¹Br signatures of formation water collected from northwest of the Algonquin Arch are distinct, with depleted isotopic values in comparison with those collected from southeast of the Arch. The δ⁸¹Br signatures of the two brines show total separation with no overlaps.

The δ⁸¹Br and δ³⁷Cl signatures of Williston Basin brines suggest several different isotopically distinct brines exist in different stratigraphic units, despite chemical similarities. The relatively wide range of δ³⁷Cl and δ⁸¹Br of the formation waters suggests that the ocean isotopic signatures were variable over geologic time. A seawater temporal curve for δ⁸¹Br and δ³⁷Cl was proposed with a larger variation of δ⁸¹Br in comparison with &delta³⁷Cl, which agrees very well with ⁸⁷Sr/⁸⁶Sr seawater variation during the same period.

In general, the use of chlorine and bromine stable isotopes can be very useful in assessing the origin and evolutionary processes involved in evolving formation waters and also in distinguishing different brines (end members).