Direct U–Pb dating of vitrinite in coal

Abstract

Uranium is often associated with organic matter forming U-organic matter complexes. Coal, as an organic rich material, is no exception but can it be dated using the uranium‑lead (U–Pb) method? We present U–Pb isotopic data, and trace elements concentrations directly measured on particles of the coal maceral vitrinite. Coal samples at three locations kilometers apart from the same Late Carboniferous bituminous coal seam in the Ningwu Coalfield, China were analyzed by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). The vitrinite reflectance of the three samples ranges from 0.69 % to 0.80 %, indicating that the coals experienced maximum paleotemperatures between 106 and 117 °C. The three samples each yielded similar lower intercept ages, and their combined dataset produced a consistent age of 116.5 ± 0.6 Ma (Lower Cretaceous), which is significantly younger than their depositional age (Late Carboniferous). Given that the U–Pb systematics in organic matter are not yet fully understood, particularly whether they behave similarly to or differently from those in minerals, we interpret the obtained age as representing the timing of coalification. The consistent age and coal rank that we obtained correspond to interpretations from previous studies about the timing of maximum paleotemperature and burial depth in the basin and the tectonic evolution of the North China Craton. We postulate that at 116 Ma, the uranium and lead associated with the organic matter became immobile when coalification was achieved, effectively locking the U–Pb system. These findings establish direct U–Pb dating of organic matter as a powerful and novel method for precisely constraining the timing of maximum thermal maturation and the absolute age of coalification in sedimentary basins.