Fossil brines preserved in the St-Lawrence Lowlands, Québec, Canada as revealed by their chemistry and noble gas isotopes

Abstract

Brines in Cambrian sandstones and Ordovician dolostones of the St-Lawrence Lowlands at Bécancour, Québec, Canada were sampled for analysis of all stable noble gases in order to trace their origin and migration path, in addition to quantifying their residence time. Major ion chemistry indicates that the brines are of Na–Ca–Cl type, possibly derived from halite dissolution. 87Sr/ 86Sr ratios and Ca excess indicate prolonged interactions with silicate rocks of the Proterozoic Grenville basement or the Cambrian Potsdam sandstone. The brines constrain a 2–3% contribution of mantle 3He and large amounts of nucleogenic 21Ne ∗ and 38Ar ∗ and radiogenic 4He and 40Ar ∗. 4He/ 40Ar ∗ and 21Ne ∗/ 40Ar ∗ ratios, corrected for mass fractionation during incomplete brine degassing, are identical to their production ratios in rocks. The source of salinity (halite dissolution), plus the occurrence of large amounts of 40Ar ∗ in brines constrain the residence time of Bécancour brines as being older than the Cretaceous. Evaporites in the St-Lawrence Lowlands likely existed only during Devonian–Silurian time. Brines might result from infiltration of Devonian water leaching halite, penetrating into or below the deeper Cambrian–Ordovician aquifers. During the Devonian, the basin reached temperatures higher than 250 °C, allowing for thermal maturation of local gas-prone source rocks (Utica shales) and possibly facilitating the release of radiogenic 40Ar ∗ into the brines. The last thermal event that could have facilitated the liberation of 40Ar ∗ into fluids and contributed to mantle 3He is the Cretaceous Monteregian Hills magmatic episode. For residence times younger than the Cretaceous, it is difficult to find an appropriate source of salinity and of nucleogenic/radiogenic gases to the Bécancour brines.