RARE-EARTH ELEMENT ANALYSIS OF BIOGENIC (CONODONT) APATITE: DESMOINESIAN (MIDDLE PENNSYLVANIAN), AND MISSOURIAN (UPPER PENNSYLVANIAN), MIDCONTINENT, USA

Conodonts from a full range of facies, including limestone and gray and black shale, were collected from the Fort Scott and Pawnee formations (Missourian) and the Dennis and Swope formations (Desmoinesian) from localities in Kansas, Missouri, and Iowa. Selected platform elements were analyzed for rare-earth element (REE) and trace element (TE) contents using UV laser ablation coupled to a high-resolution Inductively Coupled Plasma-Mass Spectrometer (ICP-MS).

Geochemical analyses of conodonts have traditionally been performed using taxonomically mixed bulk samples, which limit geochemical resolution. By contrast, analysis of individual conodonts permits generic comparisons among a variety of localities and stratigraphic intervals. The samples in this study span multiple formations with an extensive geographic spread in order to show temporal and regional variability in Pennsylvanian midcontinent ocean chemistry. This approach may shed light on the Middle and Upper Pennsylvanian processes responsible for the depositional patterns.

Microanalyses indicate that REE and TE variation may be significant within individual conodont elements, signifying that the conodonts have not undergone diagenetic geochemical resetting. This small-scale heterogeneity indicates that conodonts may be used for chemostratigraphy, both regionally and globally. Although Ce anomalies have been proposed as proxies for paleoredox conditions, preliminary data do not show systematic variation in the Ce anomaly in lithologies spanning a range of independently constrained depositional oxygen conditions. Preliminary data also indicate that conodont apatite is middle REE (MREE) enriched, as has been observed in coexisting authigenic apatite.