Radiometric dating of marine-influenced coal using Re–Os geochronology

Abstract

Coal deposits are integral to understanding the structural evolution and thermal history of sedimentary basins and correlating contemporeous estuarine and fluvial delatic strata with marine sections. While marine shales may readily lend themselves to Re-Os dating due to the dominance of hydrogenous Re and Os, the lack of a chronometer for near-shore sedimentary environments hampers basinwide correlations in absolute time. Here, we employ the Re-Os geochronometer, along with total organic carbon (TOC) and Rock-Eval data, to determine the timing and conditions of a marine incursion at the top of the Matewan coal bed, Kanawha Formation, Pottsville Group, West Virginia, USA. The observed range for hydrogen index (HI: 267-290 mg hydrocarbon/gram total organic carbon) for these coal samples suggests dominance of aliphatic hydrocarbons with low carbon (<C 19) chain length. Average Re (107.6 ± 16.4 ng/g) and Os (0.52 ± 0.09 ng/g) concentrations of the marine-influenced Matewan coal are higher by few orders of magnitude than published data for terrestrial coal. A Re-Os isochron for the Matewan coal provides an age of 325 ± 14 Ma (Model 3; MSWD = 12; n = 19; 2σ). This is the first Re-Os age derived from coal samples; the age overlaps a new composite Re-Os age of 317 ± 2 Ma for the immeditely overlying Betsie Shale Member. External precision for replicate Os analyses carried out for several Matewan coal samples shows a positive correlation with their HI. The HI, which is low in terrestrial organic matter, reflects the degree of marine influence. Thus, samples with the most profound marine influence also have the best analytical reproducibility. Equilibration of Os isotopes with seawater under marine conditions overwhelms variability inherited from terrestrial plant debris, decreasing scatter on the isochron. The 187 Re/ 188 Os ratios of the Matewan coal (~3300-5135) are higher than most of those previously published for Phanerozoic black shale (mostly <2000). Mass balance 3 calculations based on Re/TOC and Os/TOC ratios for the Matewan coal indicate that both Re and Os are primarily marine in origin, and their high 187 Re/ 188 Os ratios confirm efficient removal of both elements from a sulphidic water column into the coal. We show that Re-Os geochronology of marine-influenced coal can be a viable tool for constraining depositional ages.

Cenozoic evolution of the Qaidam basin and implications for the growth of the northern Tibetan plateau: A review

“…across the basin would not only provide independent age controls for the corresponding magnetostratigraphy studies but also bring detailed constraints on the potential diachronicity of the units inside the basin. Fourth, Re-Os geochronometry has been widely used as a proxy for determining precise and accurate depositional ages of marine organic-rich rocks (Rooney et al, 2020;Selby and Creaser, 2003;Tripathy et al, 2015) and is now successfully applied to similar deposits in lacustrine nonmarine systems (Cumming et al, 2012;Cumming et al, 2014;Meng et al, 2020). As the largest petroliferous basin in the Tibetan plateau, the Qaidam basin contains abundant rocks that are rich in organic matter, especially the lacustrine source rocks in the Xiaganchaigou Fm.…”

Time, hydrologic landscape and the long-term storage of peatland carbon in sedimentary basins

“…Given sufficiently thick coal, predicted timeframes may also be tested directly. For example using Ar-Ar radiometric ages from associated volcanic ash deposits (Chen et al, 2014), or Re-Os dates from marine-influenced coal (Tripathy et al, 2015). Uncertainty in the radiogenic age is often greater than the likely duration of the seam and this may constrain useful application.…”

New Insight into the Depositional Age of No. 6 Coal in Heidaigou Mine, Late Paleozoic Jungar Coalfield, Inner Mongolia, China

“…Coal is an organic sedimentary rock; determining its accurate depositional age will help us to reconstruct the evolution history of the sedimentary deposits [1]. In general, the magnetic stratigraphic method [2], oil generation capacity of coals [3,4], palynology of coal [5], radiometric dating of tonstein [6], and Re-Os isotopic analysis [1,7] can be used to determine coal depositional age.…”

“…Coal is an organic sedimentary rock; determining its accurate depositional age will help us to reconstruct the evolution history of the sedimentary deposits [1]. In general, the magnetic stratigraphic method [2], oil generation capacity of coals [3,4], palynology of coal [5], radiometric dating of tonstein [6], and Re-Os isotopic analysis [1,7] can be used to determine coal depositional age. Recently, Tripathy et al [1] proposed that the Re-Os isotopic analysis was a viable method of dating organic-rich sediments; whereas for terrestrial coals, the Re-Os age was not useful for determining the depositional ages because of the highly heterogeneous nature of the initial Os isotopic composition [7].…”

“…In general, the magnetic stratigraphic method [2], oil generation capacity of coals [3,4], palynology of coal [5], radiometric dating of tonstein [6], and Re-Os isotopic analysis [1,7] can be used to determine coal depositional age. Recently, Tripathy et al [1] proposed that the Re-Os isotopic analysis was a viable method of dating organic-rich sediments; whereas for terrestrial coals, the Re-Os age was not useful for determining the depositional ages because of the highly heterogeneous nature of the initial Os isotopic composition [7]. It is a challenge to obtain the direct age of non-marine-influenced coals in which there is no tonstein.…”