| Precise 187Re-187Os ages of 620.8 ± 8.1 Ma and 607.6 ± 8.3 Ma were obtained with inverse aqua regia and CrO3-H2SO4 dissolution techniques, respectively, for carbonaceous, sulfidic slates of the Old Fort Point Formation (OFP), Windermere Supergroup. The CrO3-H2SO4 method provides the most accurate determination of the depositional age and 187Os/ 188Os isotopic ratio (0.63 ± 0.05) of Neoproterozoic seawater. The OFP is believed to be the deep-water facies equivalent of carbonates that cap the Mount Vreeland and Ice Brook diamictites in the Canadian Cordillera. A minimum age constraint of ∼608 Ma is therefore inferred for the second Windermere glaciation which correlates it with other Lower Marinoan glaciogenic deposits. A geologically meaningless age (471 ± 11 Ma) was obtained for more metamorphosed slates of the stratigraphically higher Upper Miette Group. This suggests the Re-Os geochronometer may not be applicable to organic-rich mudrocks metamorphosed higher than lower-greenschist (-chlorite) grade. |
