Origin and Rb-87-Sr-87 age of porewaters in low permeability Ordovician sediments on the eastern flank of the Michigan Basin, Tiverton, Ontario, Canada

摘要

Porewater extractions and acid leachates of rock core from a 250 m thick sequence of low-permeability Ordovician-age shales and limestones, on the eastern flank of the Michigan Basin, were analysed for strontium isotope ratios in an attempt to infer porewater ages from observed Sr-87/Sr-86 enrichments. The porewaters originated as Ordovician seawater, which subsequently mixed with evaporated Silurian seawater infiltrating from above, and, to some extent, with a deep brine-with an enriched Sr-87/Sr-86 signature-from the underlying crystalline shield or deep basin. The porewater Sr-87/Sr-86 ratios are more radiogenic than contemporaneous seawater but show no obvious correlation to those leached from the solid rock phases. Accepting that the initial Sr-87/Sr-86 signatures in porewaters were dominated by Late Silurian brine, potentially with an additional deep brine component, the excess of radiogenic Sr-87 appears to represent ingrowth from Rb-87 decay over a time span of some 420 million years, approaching the depositional age of the rocks. Similarly, Rb/Sr errochron ages of acid leachates of solid phases, and the calculated initial Sr-87/Sr-86 isotopic ratios, are consistent with a proposition that the calcites inherited their Sr from Ordovician seawater and were dolomitized shortly afterwards by infiltrating Mg-enriched evaporative brine, indicating long-term conservative behaviour for the enclosing carbonate rocks. The errochron for leachates from (alumino) silicates yields a high initial Sr-87/Sr-86, but with an errochron age of about 340 +/- 48 Ma, likely owing to variable admixtures of diagenetic illite in the shales. Overall, the data provide evidence for a stable hydrologic regime since Paleozoic time.