Sr isotopic variation in single conodont elements: Implications for defining the Sr seawater curve

Sr isotopic compositions of single conodont elements were measured to investigate the potential usefulness of conodonts in defining the Sr isotope seawater curve. Analysed elements had very low colour alteration indices (CAI = 1) and were of the coniform species Drepanodus arcuatus (Pander, 1856), sampled from the Early Ordovician (early Bendigonian, ~490 - 492Ma) Emanuel Formation in the Canning Basin of Western Australia. ⁸⁷Sr/⁸⁶Sr ratios measured from single whole conodont elements were compared to those from mechanically separated cusp tissue and from the element base with attachment cone, showing significant isotopic heterogeneity within single elements. Cusps are characterised by significantly lower ⁸⁷SR/⁸⁶Sr ratios (0.708949 - 0.709056) than their associated base and attachment cone (0.709503 - 0.709829), and are well within the boundaries of the seawater curve. Single whole elements have ⁸⁷Sr/⁸⁶Sr ratios (0.709097 - 0.709282) intermediate between cusp and basal tissue. Host-rock limestone from the same horizon yielded high ⁸⁷Sr/⁸⁶Sr ratios (0.709368, 0.709454). A few small, thin-shelled articulate brachiopods recovered from elsewhere in the section have also suffered significant diagenetic alteration; the ⁸⁷Sr/⁸⁶Sr measurements of secondary shell material range from 0.709027 to 0.709430. These results indicate both carbonate and phosphate components have experienced significant diagenetic alteration, and that basal tissue appears to be a major source of secondary Sr in D. arcuatus. Despite the isotopic heterogeneity of conodont apatite, preliminary results suggest that the lowest, and perhaps primary, ⁸⁷Sr/⁸⁶Sr ratio (0.708750) is retained near the 'core' of cusp tissue, at the growth axis.