Chlorine stable isotope distribution of Michigan Basin formation waters

The Cl isotope ratio, mass³⁷Cl/³⁵Cl, was measured on 22 formation waters from Mississippian, Devonian, Silurian, Ordovician and Cambrian strata in the Michigan Basin. Because of its resistance to fractionation, the ratio was used to find evidence of mixing of formation waters within the Michigan Basin and between the Canadian Shield and the basin. The δ³⁷Cl composition of waters decreased from +0.05 to −0.55 (per mil difference from SMOC, precision of 0.16) with age of the strata among Devonian, Silurian, Ordovician and Cambrian samples from the basin margin. Mississippian samples from the middle of the basin were isotopically heaviest at +0.1. Ordovician samples, also from mid-basin, were isotopically lightest at −1.2. On plots of δ³⁷ Cl vs Cl/Br and δ³⁷ Cl vs⁸⁷Sr/⁸⁶Sr samples at the basin margin trend toward enrichment in³⁵Cl and⁸⁷Sr and increasing Cl/Br suggesting interformational mixing of the waters. On a δ³⁷ ClCl/Br plot, three samples not on this trend and tending toward high Cl/Br may reflect evaporite dissolution. Canadian Shield Waters were plotted with Michigan Basin waters on the graphs of δ³⁷ Cl vs Ca/Cl and δ³⁷ Cl vs K/Cl. On both plots data fall along linear trends of³⁵Cl depletion with Ca/Cl increase and with K/Cl decrease. Ordovician waters from the middle of the basin and shield waters are end members on the plots. The results suggest that despite water-rock interactions, δ³⁷ Cl data may be useful in studies of mixing relations in formation waters.