CHEMICAL SYSTEMATICS OF CONODONT APATITE DETERMINED BY LASER ABLATION ICPMS

Abstract

Minor and trace element compositions of a suite of Ordovician, Silurian, and Permian conodonts have been characterised by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Continuous, high-resolution chemical depth profiles through individual conodont elements reveal systematic compositional differences between the component histologies (albid, hyaline, and basal body tissues). Comparative analyses of contemporaneous bio-apatites (ichthyoliths and inarticulate brachiopods), as well as Holocene and modern fish material, show linear relationships between their respective rare earth element, yttrium, lead, thorium, and uranium compositions, which has implications for their relative permeability and susceptibility to diagenesis. Assessment of LA-ICPMS profiles in the context of histology, general morphological structure, and post-depositional chemical exchange, suggests that conodont albid crown is the least permeable histology, whereas hyaline crown, basal tissue, ichthyoliths, and inarticulate brachiopods are strongly overprinted by the selective uptake of Y-REE-Th-U during diagenesis. Conodont albid crown is therefore more resistant than other bio-apatite histologies to postmortem uptake of Y-REE-Th-U, and trends below detection limits that more closely approach primary conodont compositions. This relationship between sample histology and geochemistry, especially in the context of diagenetic overprinting, demands careful consideration when reconstructing palaeoseawater compositions. © 2006 Elsevier B.V. All rights reserved.