An isotopic survey of δ81Br and δ37Cl of dissolved halides in the Canadian and Fennoscandian Shields

Chlorine and bromine are two major anionic components of most brines, and typically behave conservatively in groundwater systems. Chlorine isotopes have been utilized to determine brine evolution during water rock evolution, very few investigations have analyzed for bromine isotopes. In this paper, brines and fluids from the Canadian and Fennoscandian Shields are characterized through a survey of chlorine and bromine stable isotopes. Stable chlorine and bromine isotopic values in Fennoscandian Shield fluids were more positive, and a greater range of values than was observed for Canadian Shield fluids. For the Fennoscandian Shield, isotopic values for δ37Cl varied between − 0.54‰ and + 1.52‰ SMOC, while δ81Br values ranged between + 0.26‰ and + 2.04‰ SMOB, while values in the Canadian Shield varied between − 0.78‰ and + 0.98‰ SMOC and + 0.01‰ and +1.29‰ SMOB, respectively. A weak positive correlation between chlorine and bromine isotopes was also observed. At one site with serpentinite rocks, a large variation in δ37Cl isotopic values compared with minimal variation in δ81Br values is attributed to ion filtration through serpentinite, which affected the Cl but not Br ions. Comparisons with other isotopic systems, such as 87Sr/86Sr, indicate water–rock interactions at some sites are likely to influence halogen isotopic composition (δ37Cl, δ81Br). The δ37Cl and δ81Br values of the investigated samples do not support a marine origin for these brines. However, if a seawater origin were to be considered for the fluids, a process or combination of processes significantly altered chlorine and bromine isotopic signatures. A positive correlation between the fluid halide isotopic composition (δ37Cl, δ81Br) and methane gas isotopic composition (δ2H, δ13C) may be due to changes in redox, pH, temperature and pressure conditions, as well as diffusion over geologic time. Although overlap occurs, the differences between the chlorine and bromine stable isotope ranges and behaviors for crystalline shields and sedimentary basins presented in this paper are significant, which indicates either different sources or different evolutionary processes in the two different environments. This could have implications to several shield evolutionary pathways published in the present literature.