CONSTRUCTION OF SECULAR SEAWATER SR ISOTOPE CURVES: COMPARISON OF 87SR/86SR ANALYSIS BY LASER ABLATION MULTICOLLECTOR ICP-MS AND TIMS

A study undertaken to evaluate the potential for constructing secular seawater Sr isotope curves by laser ablation multicollector (MC)-ICP-MS compares ⁸⁷Sr/⁸⁶Sr data obtained by TIMS and by solution and laser ablation MC-ICP-MS for Cambrian marine cements and Mississippian brachiopods and marine cements. The laser ablation ⁸⁷Sr/⁸⁶Sr ratios define a Cambrian secular Sr isotope curve that is in good agreement with that delineated by TIMS data. Mississippian samples define a linear rise in ⁸⁷Sr/⁸⁶Sr ratios that overlap the recently established European Carboniferous Sr isotope curve, and are in accord with independent biostratigraphic constraints. Analysis of cements and brachiopods was carried out on thick (100 to 150µm) sections using straight line scans and a beam size between 60 and 95µm and 80 to 100% output. Typical acquisition time is 1-2 minutes, depending on the length of lines and the preset scan speed. The ablated carbonate material is typically on the order of a few micrograms, leading to a few nanograms of analyzed Sr. Average internal precision is better than 50ppm. Kr interferences on 84 and 86 peaks were monitored and subtracted. Accuracy of measured ⁸⁷Sr/⁸⁶Sr ratios was verified by measuring the NIST Sr standard and an in-house carbonate standard, and further augmented by TIMS analyses of microdrilled samples adjacent to laser ablation spots. Heterogeneity in marine cement ⁸⁷Sr/⁸⁶Sr ratios typically covaries with submm-scale variations in cathodoluminescence suggesting that the spatial resolution afforded by laser ablation identifies diagenetic heterogeneity that cannot be resolved through microsampling and solution analysis. Laser ablation sampling of bulk micrites and marine-cemented matrix produced significantly greater heterogeneity in ⁸⁷Sr/⁸⁶Sr ratios than observed in marine cements. In contrast to pre-washing techniques applied during wet chemical preparation of samples, heterogeneity in laser ablation ⁸⁷Sr/⁸⁶Sr ratios likely reflects the inability to remove the ⁸⁷Sr contribution from trace phyllosilicates occluded within the carbonate matrix. Overall, the results of this comparison study indicates that laser ablation MC-ICP-MS has the potential for high quality, rapid construction of seawater Sr isotope curves through analysis of least altered marine cements and biogenic calcites.