Accuracy Improvement in the LA-MC-ICP-MS Measurement of 87Sr/86Sr in Bioapatite Using Dentin of Carcharinus leucas (Bull shark) to Estimate 87Rb/85Rb Instrument Induced Fractionation

In situ(87)Sr/Sr-86 microanalysis in bioapatite using laser ablation (LA-)MC-ICP-MS is an essential tool for provenance studies, as enamel tissue build-up records the regional Sr isotopic composition of ingested food and water sources. Several factors hamper the acquisition of reliable and precise Sr-87/Sr-86 data: isobaric interferences, elemental and isotopic fractionation of Rb and Sr, and the lack of certified reference materials. Here we thoroughly characterise several teeth from Carcharinus leucas (Bull shark) for the spatial distribution of Sr and Rb mass fraction by LA-ICP-Q-MS, and their Sr-87/Sr-86 ratio by solution MC-ICP-MS, MC-TIMS and LA-MC-ICP-MS, and which were used subsequently as a reference material during laser ablation measurement of Sr isotope ratios in bioapatite. We establish a protocol to estimate the Rb-85/Rb-87 mass bias from the analysis of shark teeth that demonstrates that the common assumption of equal Sr and Rb instrument induced fractionation can lead to a systematic bias in Sr-87/Sr-86 isotope ratios. Using the shark teeth as an "external" reference material to measure beta(Rb) yielded Sr-87/Sr-86 compositions in unknown samples that are within approximately -42 ppm to +66 ppm of expected values, instead of approximately -50 ppm to +190 ppm if assuming beta(Rb) = beta(Sr). Finally, this methodology was tested on fossil gomphotherid (Rhynchotherium sp.) enamel, allowing us to make preliminary inferences about its palaeobiogeography.