U–Pb systematics of an Upper Carboniferous black shale from South Yorkshire, UK

Abstract

Open-system behaviour of uraniferous shales, which has been known for many years, has discouraged attempts to use U–Pb geochronology to date sedimentary systems. Techniques now available can facilitate better understanding of their geochemical evolution and their possible use in geochronometry. For the U-rich Alton ( G. listeri) Marine Band, a combined fission track mapping, electron optical and sequential chemical extraction study confirms that uranium is incorporated into francolite, an early diagenetic precipitate. U–Pb analyses of uranium-rich (>1000 ppm) francolite nodules are discordant and imply ages ∼50–150 Ma younger than the date of sedimentation. Pb isotopic analysis suggests that uranium daughters continually leaked from the francolite, 238U daughters being released more efficiently than those of 235U. Extrapolation of the U–Pb data to concordia produces an age consistent with the time of sedimentation. These features are also displayed by other uranium-rich shales such as the Swedish Kolm Measures, despite uranium being incorporated into different phases. Preferential loss of 238U daughters from fine-grained particles due to alpha recoil could explain the unusual U–Pb isotopic composition, in both examples. Further work would be justified to investigate the application of U–Pb isotopic analysis of such material to date sedimentary sequences.