Accuracy of laser-ablation (LA)-MC-ICPMS Sr isotope analysis of (bio)apatite – a problem reassessed

Abstract

Apatite is a key mineral whose Sr-isotope record has a wide range of applications including palaeofluid flow studies from inorganic apatite, and past faunal/human mobility or palaeoecology using bioapatite. The incremental growth of mammalian enamel bioapatite potentially allows extraction of Sr isotopic compositions at sub-annual time-resolution using laser-ablation plasma mass spectrometry (LA-MC-ICPMS). However, existing apatite LA-MC-ICPMS Sr-isotope data have yielded mixed results. Here we assess the achievable accuracy/precision of (bio)apatite LA-MC-ICPMS Sr-isotope analysis and evaluate sources of inaccuracy. Using robust plasma conditions (ThO⁺/Th⁺ < 0.2%), we obtain long-term (4 year) accurate and precise Sr-isotope data for modern shark teeth for both radiogenic ⁸⁷Sr/⁸⁶Sr (0.709171 ± 0.000053, 2 SD), and naturally invariant ⁸⁴Sr/⁸⁶Sr (0.056500 ± 0.000040, 2 SD). Based on our accurate ⁸⁴Sr/⁸⁶Sr-results also for low-Sr enamel, we deduce that interferences are successfully corrected (Kr) or negligible (Ca-argide/dimer), leaving ⁸⁷Rb and ⁴⁰Ca³¹P¹⁶O isobaric interferences as key potential sources for ⁸⁷Sr/⁸⁶Sr inaccuracy. Our (pseudo)high-resolution mass scans using a virtually Rb–Sr-free, concentrated Ca–P-solution simulating apatite LA analysis show no evidence for ⁴⁰Ca³¹P¹⁶O at the required intensities to explain previously observed ⁸⁷Sr/⁸⁶Sr offsets. Rather, using the same Ca–P solution with varying Rb + Sr isotope standard additions, we accurately constrain the mass bias-corrected ⁸⁵Rb/⁸⁷Rb-ratio, and using apatite glasses assess the extent of Rb/Sr elemental fractionation during laser-ablation. Finally, we present concordant LA-MC-ICPMS and microsampled-TIMS ⁸⁷Sr/⁸⁶Sr results for low-Sr tooth enamel with highly variable inter & intra-⁸⁷Sr/⁸⁶Sr ratios and comparatively high Rb/Sr-ratios. This archaeological example also illustrates well the problem of defining equivalent sample volumes that allow unequivocal comparison between LA and TIMS data.