Why terrestrial coals cannot be dated using the Re-Os geochronometer: Evidence from the Finnmark Platform, southern Barents Sea and the Fire Clay coal horizon, Central Appalachian Basin

This study is the first attempt to apply the Re-Os geochronometer on terrestrial coals. In case of the Soldogg coals and shale, the 187Re/188Os vs. 187Os/188Os relationship displays scatter with no discernible linear trend. The scatter is ascribed to heterogeneity in the initial 187Os/188Os ratios (Osi) and lack of post-depositional isotopic equilibration during subsequent diagenesis. For the Fire Clay coals, 187Os/188Os and 187Re/188Os are well correlated; linear regression of the data, however, does not yield a geologically reasonable age. Rather, the linear relationship results from binary mixing between two end-members: The Fire Clay coal and tonstein, suggesting that Re-Os systematics of terrestrial coals depositing very close to tonstein beds can be significantly disturbed. We conclude that the post-depositional equilibration with a large Re-Os reservoir, as with a marine incursion, is necessary to record a meaningful Re-Os radiometric age for the coals.