SOLUTION-LASER ABLATION MC-ICPMS COMPARISONS OF CONODONT APATITE FOR 87SR/86SR

The ⁸⁷Sr/⁸⁶Sr composition of fossil calcites and apatites enables reconstruction of paleoseawater ⁸⁷Sr/⁸⁶Sr, which may constrain chronostratigraphy and paleoenvironmental conditions. Bioapatites, including conodont microfossils, require Sr to be purified via anion exchange columns prior to analysis by MC-ICPMS or TIMS methods. The laborious and expensive nature of conventional solution measuremements, as well as the requirement of homogenzation of large quantities of bioapatite, inspired researchers to analyze conodonts for ⁸⁷Sr/⁸⁶Sr by laser ablation (LA) MC-ICPMS. However, this method for determining conodont ⁸⁷Sr/⁸⁶Sr has not been calibrated to the established solution MC-ICPMS method. We compared aliquots of conodonts from the same stratigraphic height for ⁸⁷Sr/⁸⁶Sr composition by solution and laser ablation analysis using MC-ICPMS. Analysis of samples of basal platforms and albid cones, as well as a range of preservation conditions, enabled evaluation of the potential of chemical heterogeneity on the comparison results. In order to decrease the ⁴⁰Ca³¹P¹⁶O⁺ interference on mass 87 during LA analyses, plasma sampling depth was increased and a small amount of N₂ was added to the Ar sample gas. ⁸⁷Sr/⁸⁶Sr analysis of modern fish bioapatite verified this approach. Medium to high resolution settings may resolve the ⁴⁰Ca³¹P¹⁶O⁺ interference in analysis of apatite with more abundant Sr. Preliminary results reveal significantly greater ⁸⁷Sr/⁸⁶Sr ratios of laser ablation values compared to solution values. This enrichment generally correlates with Yb/Sr and Er/Sr in the conodonts. As such, rare earth elements (REEs), which accumulate in biological apatite during post-mortem recrystallization, may form doubly-charged spectral interferences on the masses measured for ⁸⁷Sr/⁸⁶Sr. The inteferences may accurately be determined by monitoring the half masses of Yb and Er isotopes (83.5, 85.5, 86.5) using an ion counter and assuming natural abundance ratios of the REE isotopes. Conodonts measured for ⁸⁷Sr/⁸⁶Sr by this method yield results comparable to conventional solution analyses. This LA MC-ICPMS technique may aid in the rapid construction of seawater ⁸⁷Sr/⁸⁶Sr curves to improve chemostratigraphic correlations and paleoceanographic reconstructions.