URANIUM ISOTOPE VARIATIONS IN SHALE AND CARBONATE OF THE MIDDLE DEVONIAN MARCELLUS FORMATION, PENNSYLVANIA, USA

The recent boom in natural gas extraction from shales of the Middle Devonian Marcellus Formation in the eastern USA has prompted an interest in the depositional conditions and subsequent diagenetic history of the unit. Recent work has shown that uranium (U) isotope fractionation can be an indicator of local and global redox conditions during deposition of shale and carbonate (Montoya-Pino et al. 2010, Geology 38, 315; Brennecka et al. 2011, PNAS 108, 17631). Here we report preliminary U isotope data from Marcellus Formation core samples from Greene County, southwestern Pennsylvania.

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.