87Sr/86Sr,143Nd/144Nd and REEs in Silurian phosphatic fossils

The feasibility of using the chemistry of phosphatic fossil material to trace palaeo-seawater composition for the Silurian has been evaluated. Contrary to Recent material, fossil fish are shown to be unreliable for Sr isotope stratigraphy. Conodonts, however, may be used by employing 0.2M HNO3 dissolution, except for samples showing degrees of thermal maturation greater than colour alteration index (CAI) values of 2.5. Samples from the same stratigraphic levels from the U.K., Czechoslovakia, Gotland and the U.S.A. gave similar87Sr/86Sr ratios, suggesting that they have recorded the original seawater value. Significant inter-specific differences occur in REE concentrations and Nd isotopic compositions of conodont elements, and Ce and Eu anomalies vary with REE content. This suggests that artefacts are introduced at the same time that large amounts of REEs are incorporated into the apatite. Preliminary results for the Sr isotopic composition of Silurian seawater are presented. Values are generally lower than previously suggested, showing an increase in87Sr/86Sr from ∼ 0.70825 at 435 Ma rising to ∼ 0.70875 at 418 Ma and levelling off in the Late Silurian. The overall rate of increase in seawater ratio is about2–3 × 10−5 Ma−1 and can be accounted for by the widespread decrease in volcanism compared with the Ordovician; a proportionally greater flux of Sr to the oceans from continental erosion of only ∼ 10% would account for the long-term increase in seawater87Sr/86Sr ratio throughout the Silurian.