URANIUM ISOTOPE VARIATIONS IN SHALE AND CARBONATE OF THE MIDDLE DEVONIAN MARCELLUS FORMATION, PENNSYLVANIA, USA
Sample depths ranged from 2,374 to 2,407 m and encompass both shale and carbonate units of the Marcellus Formation. Uranium isotope data were obtained from whole rock samples (including carbonate, silicate, and organic matter). The acetic acid-soluble (carbonate) fraction, determined on separate splits of the samples, ranged from 15-73%. Rock powders were dissolved using a combination of HNO3, HF, HClO4, and H2O2, and U was subsequently separated and run by MC-ICP-MS using standard bracketing methods. Uranium concentrations range from 10 to 126 µg/g, and do not significantly correlate with carbonate content. 238U/235U ratios fall between 137.80 and 137.90, within the range of previously reported shale data, with the exception of the lowermost, carbonate-rich sample, which yielded a higher value. In all cases, the 234U/238U activity ratio is near unity. Carbonate in this core could represent primary deposition and/or diagenetic dissolution/precipitation. Sequential extraction experiments are underway to isolate uranium from different components of the rock units in order to better understand U isotope fractionation during deposition and diagenesis of the Marcellus Formation.