An isotopic survey of delta Br-81 and delta Cl-37 of dissolved halides in the Canadian and Fennoscandian Shields

作者：Stotler Randy L^*; Frape Shaun K; Shouakar Stash Orfan

来源：Chemical Geology, 2010, 274(1-2): 38-55.

DOI：10.1016/j.chemgeo.2010.03.014

• Isotopes
• Chlorine
• Bromine
• Strontium
• Methane
• Fluid evolution

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 delta Cl-37 varied between -0.54 parts per thousand and +1.52 parts per thousand SMOC, while delta Br-81 values ranged between +0.26 parts per thousand and +2.04 parts per thousand SMOB, while values in the Canadian Shield varied between -0.78 parts per thousand and +0.98 parts per thousand SMOC and +0.01 parts per thousand and +1.29 parts per thousand SMOB, respectively. A weak positive correlation between chlorine and bromine isotopes was also observed. At one site with serpentinite rocks, a large variation in delta Cl-37 isotopic values compared with minimal variation in delta Br-81 values is attributed to ion filtration through serpentinite, which affected the Cl but not Br ions. Comparisons with other isotopic systems, such as Sr-87/Sr-86, indicate water-rock interactions at some sites are likely to influence halogen isotopic composition (delta Cl-37, delta Br-81). The delta Cl-37 and delta Br-81 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 (delta Cl-37, delta Br-81) and methane gas isotopic composition (delta H-2, delta C-13) 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.

• 出版日期2010-6-15