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

Abstract

Coal horizons in terrestrial sedimentary sections are key to understanding the structural and thermal evolution of sedimentary basins. Precise and accurate age information for coal horizons can define correlations and the temporal evolution of sedimentary successions. While the biostratigraphy for coal horizons yields relative ages, tied to radiometric ages through the continuously refined geological time scale, direct radiometric ages of coal horizons have been derived only from tonsteins, lithified ash beds. The Re-Os geochronometer has been proven to record accurate and precise depositional ages for sedimentary rocks rich in organic matter. However, the reliability of the Re-Os geochronometer has not yet been assessed for coals deposited in strictly terrestrial environments. Here, in order to test the Re-Os geochronometer, we measure the Re, Os concentrations and isotopic composition of terrestrial coals from two distinct environments: the Finnmark Platform, southern Barents Sea and the Central Appalachian Basin (CAB). Samples from the Finnmark platform (Soldogg Formation) also include an adjacent organic-rich shale horizon. The shale and two coal horizons were also analysed for total organic carbon (TOC) and Rock-Eval data. Results show that the Soldogg coals and shale reached the oil generation widow (Tmax = 448 ± 2 °C), with Type III or mixed Type II/III organic matter, supporting the model for deposition in overbank deposits in a fluvial setting. In contrast, the Fire Clay coals in the CAB were deposited in a broad expanse of mires. Average Re (0.5 ± 0.4 ng/g) and Os (23 ± 26 pg/g) concentrations of terrestrial Soldogg and Fire Clay coals are similar to those of terrestrial Maghara coal and a few orders of magnitude lower than the marine-influenced Matewan coal. Elevated Re/Os* ratio (Os* = Common Os; Os corrected for ingrowth of 187Os since deposition of the sample) for the Fire Clay coals suggests highly reducing and/or euxinic conditions prevailing in the water column of the extensive Carboniferous peat-forming mire in which these coals were deposited. In contrast, lower Re/Os* for Soldogg coals highlight less reducing conditions, likely in a short-lived mire in a dynamic braided stream system. Re and Os* concentrations of terrestrial coals are on the order of Re and Os of plant materials and river sediments, suggesting that debris from plant materials and weathered continental crust are significant contributors of Re and Os to terrestrial coals, with some contribution from water column of the coal mire.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.