Origin and evolution of waters from Paleozoic formations, Southern Ontario, Canada: Additional evidence from δ37Cl and δ81Br isotopic signatures

摘要

The δ37Cl and δ81Br isotopic signatures for selected water samples from southern Ontario sedimentary formations (Cambrian through Devonian) were determined and interpreted in combination with existing chemical and isotopic information. The chemical composition and the isotopic signatures of the brines suggest that they originated from paleoseawater by evaporation. Most water samples examined in this study are brines with TDS values above 100 g/L. The only exceptions are Devonian formation waters sampled from shallow depths at the margin of the Michigan Basin, which are characterized by lower TDS values (＜ 100 g/L), and which show evidence of mixing with either meteoric waters or cold climate waters. The δ37Cl and δ81Br values of the formation waters show variations between -1.04‰ and +1.14‰ relative to SMOC and between -0.95‰ and +2.31‰ relative to SMOB, respectively. The δ37Cl range is within the known variation for chlorine stable isotopes of formation waters. When the waters are grouped according to the geological age and general lithology of the formations from which they were sampled, each group demonstrates a distinct range of δ37Cl and δ81Br signatures. Within each group, trends in both isotopes are observed as a function of geographical location (e.g., centre versus margins of the basin, or margin of Michigan versus margin of Appalachian basin), and sampling depth, which increases from the margins towards the centre of the basin.