CHLORINE STABLE-ISOTOPE DISTRIBUTION OF MICHIGAN BASIN FORMATION WATERS

The Cl isotope ratio, mass Cl-37/Cl-35, was measured on 22 formation w aters from Mississippian, Devonian, Silurian, Ordovician and Cambrian strata in the Michigan Basin. Because of its resistance to fractionati on, the ratio was used to find evidence of mixing of formation waters within the Michigan Basin and between the Canadian Shield and the basi n. The deltaCl-37 composition of waters decreased from +0.05 to -0.55 (per mil difference from SMOC, precision of 0.16) with age of the stra ta 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 , weft isotopically lightest at - 1.2. on plots of deltaCl-37 vs Cl/Br and deltaCl-37 vs Sr-87/Sr-86 samples at the basin margin trend towar d enrichment in Cl-35 and Sr-87 and increasing Cl/Br suggesting interf ormational mixing of the waters. On a deltaCl-37-Cl/Br plot, three sam ples not on this trend and tending toward high Cl/Br may reflect evapo rite dissolution. Canadian Shield Waters were plotted with Michigan Ba sin waters on the graphs of deltaCl-37 vs Ca/Cl and deltaCl-37 vs K/Cl . On both plots data fall along linear trends of Cl-35 depletion with Ca/Cl increase and with K/Cl decrease. Ordovician waters from the midd le of the basin and shield waters are end members on the plots. The re sults suggest that despite water-rock interactions, deltaCl-37 data ma y be useful in studies of mixing relations in formation waters.