Examining Dissolution Procedure Effects on Trace and Rare Earth Element Concentrations and U-Pb Ages in Conodonts

The ability to quantitatively date carbonate rocks and shales has eluded geologists due to the paucity of minerals required for geo- and thermochronometry. A small number of investigations have shown that the highly crystalline bio-apatite of conodonts, ubiquitous marine microfossils, has potential for radiometric dating. Although promising, ages calculated in previous studies are commonly dispersed and are often too old. These results suggest open-system behavior, but the cause is not known. This study evaluates whole-rock dissolution procedures and their impact on geochemical data derived from conodonts through laboratory experiments and laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS). Our hypothesis is that the dissolution of host limestone used to liberate conodonts leaches parent isotopes from the conodont elements and may be the cause of inaccurate and dispersed ages.

To test this hypothesis, large samples (20-25 kg) were collected from two well-dated units: the Ordovician Simpson Group in Oklahoma and Pennsylvanian Marmaton Group in Missouri. Limestones from these units are known to be rich in conodonts, and the individual elements are large enough for analysis. The bulk samples were crushed and mixed, and then divided into splits and digested in buffered and unbuffered acetic acid, with hold times varying from hours to two weeks. Liberated conodonts, from sample splits, will be mounted on tape and in epoxy, and analyzed using LA-ICPMS. Tape-mounted whole conodont specimens will be analyzed for rare earth and trace element concentrations using depth profiles. U-Pb ages will also be calculated from spot analyses collected in transects across epoxy-mounted specimens. These data will be used to test if time of exposure to acid during processing, tissue type (hyaline or albid), and/or element morphology (e.g., narrow coniform elements vs. more robust elements) explain observed gradients in elemental abundance from the surface to the center of elements and the accuracy and reproducibility of ages. The results will provide insights into the viability of conodonts as chronometers and their reliability as geochemical proxies.