FACTORS AFFECTING 87SR/86SR SIGNAL PRESERVATION IN CONODONT APATITE: A LONG-RANGING DATA COMPILATION AND A CASE STUDY FROM THE ORDOVICIAN OF BALTOSCANDIA

Conodont apatite is ubiquitous in Paleozoic shallow- to deep-water carbonate sediments and has served as a crucial archive of seawater ⁸⁷Sr/⁸⁶Sr. However, some datasets have reported diagenetically altered ⁸⁷Sr/⁸⁶Sr values in apparently well-preserved conodont samples, highlighting a lack of understanding of the factors that affect ⁸⁷Sr/⁸⁶Sr signal preservation in conodont apatite. We demonstrate this discrepancy here with ⁸⁷Sr/⁸⁶Sr measurements of Middle–Late Ordovician conodont apatite from two classically studied sections in the Siljan District of central Sweden. Strata of the Siljan District contain excellently preserved conodonts (Conodont Alteration Index (CAI) values of 1–2) indicating that this region underwent a relatively shallow burial regime. Conodont samples were pre-leached in weak acid to remove contaminant Sr in the surficial layer following Holmden et al. (1996, EPSL) prior to digestion and further sample preparation for measurement of ⁸⁷Sr/⁸⁶Sr by thermal ionization mass spectrometry. ⁸⁷Sr/⁸⁶Sr values from the Kårgärde section spanning the Middle–Late Ordovician (E. suecicus to P. anserinus conodont zones) fall from 0.70880 to 0.70855, showing generally good agreement with the Laurentian seawater ⁸⁷Sr/⁸⁶Sr curve for this interval. The Fjäcka section, located only 20 km from Kårgärde, spans the early Late Ordovician (A. tvaerensis to A. superbus conodont zones) and shows ⁸⁷Sr/⁸⁶Sr values increasing upsection from ~0.70820 to 0.70845, which is ~0.003–0.006 greater than established seawater values of this age, indicating considerable influence from diagenetic alteration. The reasons for the difference in conodont ⁸⁷Sr/⁸⁶Sr preservation between these two sections having similar lithologies and inferred burial histories are still unclear. We compare these measurements to preliminary conodont ⁸⁷Sr/⁸⁶Sr measurements from the Uuga Cliff section in Estonia, which show Middle Ordovician (P. originalis to P. anserinus conodont zones) ⁸⁷Sr/⁸⁶Sr values decreasing from 0.70876 to 0.70850 in good agreement with Kårgärde and Laurentian values. We will also present the results of an ongoing effort to build a database of published conodont ⁸⁷Sr/⁸⁶Sr to evaluate possible relationships between host rock lithology, CAI, and other parameters and ⁸⁷Sr/⁸⁶Sr signal preservation.