RARE-EARTH AND TRACE ELEMENT ANALYSIS OF BIOGENIC (CONODONT) APATITE: DENNIS AND SWOPE FORMATIONS, UPPER PENNSYLVANIAN (MISSOURIAN), MIDCONTINENT, USA

Pennsylvanian strata of midcontinent North America are characterized by cyclic deposits of inferred glacioeustatic origin. The classic cyclothem spans from nearshore siliciclastics to limestone to offshore euxinic shale. Conodonts from this full range of facies were collected in two Missourian cyclothems, the Swope and Dennis formations of Iowa, Missouri, Kansas, and Oklahoma, and were analyzed for rare-earth element (REE) and trace element (TE) contents. Data were generated with a threefold agenda: (1) to characterize local and global ocean chemistry as expressed in a dynamic paleoepicratonic seaway; (2) to use stratigraphic and spatial trends to better constrain the mechanisms that drove cyclic deposition; and (3) to test new approaches to conodont geochemistry and the reliability of preserved primary records relative to diagenetic overprints, including apatite oxygen isotope data.

Preliminary results generated by UV laser ablation coupled to a high-resolution Inductively Coupled Plasma Mass Spectrometer suggest that REE and TE variation may be significant within individual conodont elements. Such small-scale heterogeneities argue against wholesale diagenetic resetting. To test the extent to which biogenic apatite undergoes alteration during diagenesis and the potential for recording primary (water-column) variation that occurred during the life span of a given individual, growth lamellae within single conodont platform elements will be analyzed by laser for REE and TE concentrations. Geochemical analyses of conodonts have traditionally been performed using taxonomically mixed bulk samples, which limits geochemical resolution. By contrast, analysis of individual conodonts will permit intrageneric comparisons among a variety of localities and stratigraphic intervals. Such comparisons should illuminate temporal and regional variability in midcontinent ocean chemistry during the Pennsylvanian. Furthermore, intergeneric REE and TE variation in a given stratigraphic horizon may reflect vertical chemical gradients within the ancient water column. More broadly, this presentation is a progress report of approaches to conodont chemostratigraphy.