New Re–Os isotopic data were obtained from chlorite-grade black shales from the upper Old Fort Point Formation (Windermere Supergroup), a post-glacial Neoproterozoic marker horizon in western Canada. A Re–Os isochron date of 634±57 Ma (MSWD=65, n=5) was determined using the conventional inverse aqua regia digestion medium. However, dissolution of the same samples with a new CrO3–H2SO4 dissolution technique [Chem. Geol. 200 (2003) 225] yielded a much more precise date of 607.8±4.7 Ma (MSWD=1.2). Both dates are in agreement with existing U–Pb age constraints that bracket the Old Fort Point Formation between ∼685 and ∼570 Ma. The distinctive Re–Os systematics recorded by the two analytical protocols is explained by dissolution of a variably radiogenic, detrital Os component by the aqua regia method. In contrast, the CrO3–H2SO4 technique minimizes this detrital component by selectively dissolving organic matter that is dominated by hydrogenous (seawater) Re and Os. The date of 607.8±4.7 Ma is thus interpreted as the depositional age for the upper Old Fort Point Formation providing a minimum age constraint for the timing of the second Windermere glaciation in western Canada. This ice age is correlative with the Marinoan (∼620–600 Ma) ice age and older than the ∼580-Ma Gaskiers glaciation of northeastern North America. The new Re–Os age determined from the CrO3–H2SO4 digestion technique thus provides further support to a growing body of evidence for a global Marinoan glacial episode. Such an interpretation would not be discernable from the imprecise Re–Os date obtained with the aqua regia protocol. These results also indicate the potential for Re–Os radiometric dating of black shales that was not previously recognized. Importantly, neither chlorite-grade metamorphism nor the low organic content (TOC <1%) of the Old Fort Point Formation precluded the determination of a precise Re–Os depositional age using the CrO3–H2SO4 analytical protocol.