RARE-EARTH ELEMENT REDISTRIBUTION AND ITS EFFECTS ON THE NEODYMIUM ISOTOPE SYSTEM IN THE AUSTIN-GLEN MEMBER OF THE NORMANSKILL FORMATION, NEW-YORK, USA

Three groups of REE patterns are observed for the sandstones and shales of the Middle Ordovician Austin Glen Member of the Normanskill Formation. One sandstone sample is enriched in middle and light rare earth elements. The majority of samples are light rare earth element (LREE) enriched with negative Eu-anomalies and flat heavy rare earth elements (HREEs), whereas a third group appears to have lost LREEs and middle rare earth element (MREEs). Homogeneous neodymium isotopic compositions for all samples at about the time of deposition, epsilon(Nd) (470 Ma) = -8.2 +/- 1.1(2SD), indicate that the source of the Austin Glen Member was well mixed and that the neodymium had a long-term enriched history. The differences in the shape and abundances of the REE patterns combined with the neodymium isotope characteristics of the samples lead us to the conclusion that samarium and neodymium were fractionated and that the neodymium isotopic system might have been disturbed at about the time of deposition (470 Ma). Most samples appear to have been affected by this process. The two most altered shales lost about half of their neodymium and their Sm/Nd ratios are higher than the Sm/Nd ratios of average shales. These two shales give clearly erroneous mantle-extraction ages, but the average T(DM) of the provenance can still be deduced (1700-1800 Ma) from the least disturbed sandstones with normal upper crustal Sm/Nd ratios. Accordingly, this study demonstrates that the REEs may be transported and neodymium isotopes may be reequilibrated under certain sedimentary conditions (e.g., diagenesis). However, in spite of evidence for REE redistribution, provenance information (T(DM), original REE patterns) may still be inferred.