Geologic variability of conodont strontium isotopic composition quantified by laser ablation multiple collection inductively coupled plasma mass spectrometry
Julie M. Griffin, Isabel P. Montañez, Justin Glessner, Jitao Chen, Malte Willmes
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.