Geologic variability of conodont strontium isotopic composition quantified by laser ablation multiple collection inductively coupled plasma mass spectrometry

Highlights

• Medium mass resolution LA MC-ICP-MS of conodont ⁸⁷Sr/⁸⁶Sr yields values equivalent to solution.

• Duplicate conodont LA MC-ICP-MS analyses exhibit ⁸⁷Sr/⁸⁶Sr variability homogenized by solution.

• LA MC-ICP-MS analysis of conodonts is rapid, accurate, and reveals geologic ⁸⁷Sr/⁸⁶Sr variation.

Abstract

Conodont microfossils record seawater strontium isotope values (⁸⁷Sr/⁸⁶Sr), permitting chemostratigraphic correlation for tectonic and climatic reconstructions of the Paleozoic and early Mesozoic (541–201 Ma). Laser ablation multiple collection inductively coupled plasma mass spectrometry (LA MC-ICP-MS) can provide rapid, high spatial resolution ⁸⁷Sr/⁸⁶Sr analysis of conodont bioapatite but has not been validated by comparison with solution analysis. Validation of LA MC-ICP-MS should be completed in order to use the conodont ⁸⁷Sr/⁸⁶Sr values for age-correlations and environmental interpretations. Here, for the first time, we compare solution and LA MC-ICP-MS ⁸⁷Sr/⁸⁶Sr analyses of Carboniferous-age conodonts. Furthermore, we use quadrupole LA ICP-MS to determine concentrations of trace elements potentially responsible for isobaric interferences. Using increased mass resolving power (m/∆m = ~7500) and analyzing conodont tissue with low ⁸⁵Rb/⁸⁸Sr (< 0.001), we find laser ablation copacetic with solution ⁸⁷Sr/⁸⁶Sr values. The two-standard-deviation of these LA ⁸⁷Sr/⁸⁶Sr ratios (average 2SD = 0.00105) are within the two-standard-error uncertainty of solution measurements (average 2SE = 0.00001) on conodonts from the same stratigraphic level. The LA measurements are at a higher spatial resolution and on average 0.00015 higher than solution measurements. Uncertainty of the mean calculations, made on duplicate LA MC-ICP-MS analyses of individual conodonts from the same stratigraphic level, exhibit ⁸⁷Sr/⁸⁶Sr variability beyond the precision of reference materials (2SE = 0.00001). This finding suggests that solution ⁸⁷Sr/⁸⁶Sr values determined by dissolving multiple conodonts are homogenizing the conodont ⁸⁷Sr/⁸⁶Sr signal. As such, the precision of these solution measurements does not capture the geologic variability of conodont ⁸⁷Sr/⁸⁶Sr within a stratigraphic level, which may originate from differential diagenetic alteration. Conodont ⁸⁷Sr/⁸⁶Sr measurements that do not account for this variability are at risk of false calibrations with the paleo-seawater ⁸⁷Sr/⁸⁶Sr curve, which has implications for the timing of geologic events and reconstructions of paleo-environmental changes.