Constraints on the timing of Marinoan ``Snowball Earth'' glaciation by 187Re-187Os dating of a Neoproterozoic, post-glacial black shale in Western Canada

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 CrO ₃-H ₂SO ₄ 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 CrO ₃-H ₂SO ₄ 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 CrO ₃-H ₂SO ₄ 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 CrO ₃-H ₂SO ₄ analytical protocol.