Examining dissolution procedure effects on trace and rare earth element concentrations and U/Pb ages in conodonts
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The ability to quantitatively date carbonate rocks and shales using geo- and thermochronometry has eluded geologists due to the scarcity of requisite minerals. A small number of investigations have shown that the highly crystalline bio-apatite of conodont marine microfossils has potential for radiometric dating. Although a possibility, ages calculated in previous studies are commonly dispersed without a consistent age 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 units: the Middle Ordovician West Spring Creek Formation in Oklahoma and Mississippian Pierson Limestone 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 unbuffered acetic acid, with hold times varying from hours to as long as a month. Liberated conodonts, from sample splits, were mounted on tape and in epoxy, and analyzed using LA-ICPMS. Tape-mounted whole conodont specimens were analyzed for trace and rare earth element (REE) concentrations using depth profiles. U/Pb ages were determined from spot analyses collected in transects across epoxy-mounted specimens. Depth profiles exhibited patterns that portrayed both elemental loss and gain through the varying hold times. The most stable isotope was strontium in the platforms, blades, and bars. Yttrium was affected the most, while the REEs were consistently affected between morphologies. U/Pb ages were highly discordant in both samples with spot dates in Ordovician sample ranging from 220.2 +/- 84.2 Ma to 4672.9 +/- 30.6 Ma and spot dates in Mississippian sample ranging from 185 +/- 227 Ma to 4806 +/- 112 Ma. Since conodonts are a phase containing elements that can be measured, the results may have implications for geochemical data derived from certain histological and morphological types. These results also undermine confidence in the viability of conodonts as geo- and thermochronometers, and suggest that dissolution procedures can affect ages.
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M.S.