MIDDLE RARE-EARTH ENRICHMENT IN PENNSYLVANIAN CONODONTS AND IMPLICATIONS FOR PALEOENVIRONMENTAL RECONSTRUCTIONS

Conodonts from a full range of facies, including limestone and gray and black shale, were collected from the Fort Scott and Pawnee formations (Desmoinesian) and Swope and Dennis formations (Missourian) from localities in Kansas, Missouri, and Iowa. Geochemical analyses of conodonts have traditionally been performed using taxonomically mixed bulk samples, which limits geochemical resolution. By contrast, analysis of individual conodonts permits genus-level comparisons among a variety of localities and stratigraphic intervals. Selected platform elements were individually analyzed for rare-earth element (REE) and trace element (TE) contents using Laser Ablation-Inductively Coupled Plasma-Mass Spectrometer (LA-ICP-MS).

Conodont data show that different genera analyzed from the same horizon have the same REE pattern, although concentrations may differ slightly. Similarly, there is no systematic temporal or regional variation, despite the wide variety of facies represented in the Pennsylvanian rocks. Data indicate that conodont apatite is enriched in middle REE (MREE), as has been observed in coexisting nodular authigenic apatite and phosphatic black shales. The MREE enrichment is present in nearly all of the samples, regardless of lithology. Statistical methods indicate that the more progressively enriched a conodont is in REE relative to crustal levels, the greater the MREE enrichment. MREE abundance does not vary systematically with region, locality, lithology, member, or formation. Collectively, these observations confirm that MREE enrichment, and thus much of the REE record, is diagenetic. These secondary overprints invalidate the use of REE patterns in Pennsylvanian conodonts as paleoceanographic proxies, including the Ce anomaly as a measure of paleoredox.