A NEODYMIUM ISOTOPE INVESTIGATION OF SEDIMENT SOURCES FOR THE MIDDLE DEVONIAN MARCELLUS FORMATION, PENNSYLVANIA, USA
Preliminary neodymium (Nd) isotope data were obtained on a set of samples from a core extracted in Greene County, southwestern Pennsylvania, that spans a depth range of ~25 m (2374-2398 m). The samples range from marly limestone to calcareous shale to organic-rich black shale. Shales within the Marcellus Formation can have high calcium carbonate contents that can reflect post-depositional/diagenetic precipitation. To isolate the isotope systematics and understand provenance of the silicate and organic portions of the shale, carbonate was removed using acetic acid. The acetic acid-soluble portions of shales in this section of the core ranged from 15 to 39%. The residues, consisting primarily of clastic silicate minerals and organic matter, yield εNd(T) values within a tight range of -7.4±0.4 when corrected to 390 Ma (Middle Devonian). Depleted mantle model ages for the residue samples range from 1.4 to 1.6 Ga, consistent with an earlier analysis of a Marcellus outcrop sample from New York State (Caeser et al., 2010, GSA NE/SE Sect. Abstr. 111-3), and with a sediment source dominated by orogenic highlands to the east (Patchett et al., 1999, Science 283, 671-673). Ashes within this section contain inherited zircon cores which have U-Pb ages of about 1 Ga, suggesting a contribution from Grenville basement. Dy/Yb ratios between 1.5 and 1.8 are consistent with a similar source material for these layers. The tight clustering of εNd(T) values and Sm/Nd ratios suggest that post-depositional disturbance to the organic and clastic portion of the shale lithologies from this core has been minimal. Additional work is focusing on the Sr/Nd isotope contributions from exchangeable sites and carbonate, and their relationship to Marcellus produced water.