Analytical strategies for ²⁰⁷ Pb ∕  ²³⁵ U carbonate geochronology

In carbonate U–Pb geochronology, unknown initial ²³⁴ U /  ²³⁸ U disequilibria can be debilitating for the accuracy of dates relying on the ²⁰⁶ Pb /  ²³⁸ U clock. Even if residual ²³⁴ U /  ²³⁸ U disequilibrium can be measured precisely and accurately enough to perform a correction, beyond ca. 1.5 Ma (depending on the magnitude of initial excess/deficit of ²³⁴ U) the expansion of uncertainties due to the disequilibrium correction may make carbonate ²⁰⁶ Pb /  ²³⁸ U isochron dates and Tera-Wasserburg intercept dates prohibitively imprecise. An alternative approach utilising only the ²⁰⁷ Pb /  ²³⁵ U system is in principle more accurate, however the lower abundance of ²³⁵ U and ²⁰⁷ Pb relative to ²³⁸ U and ²⁰⁶ Pb inevitably results in larger analytical data-point uncertainties. Here we explore analytical strategies that maximise the potential of the ²⁰⁷ Pb /  ²³⁵ U system for carbonate geochronology. ID-TIMS and LA-ICPMS are considered the most useful, and complementary, techniques. ID-TIMS offers the ultimate precision and accuracy combined with the ability to filter data for contamination using ²⁰⁸ Pb /  ²⁰⁴ Pb, but it requires mg-sized samples that limit the spatial resolution and the spread of data along the isochron. Optimised LA-ICPMS with large spot sizes may be a quick alternative with better spatial control and enhanced spread of data along the isochron, but ²⁰⁸ Pb normalisation imposed by the difficulty of accurately analysing ²⁰⁴ Pb may add noise to the results. Overall, precise ²⁰⁷ Pb /  ²³⁵ U carbonate geochronology is feasible and with minor analytical adjustments it can be adopted broadly instead of the ²³⁸ U-based methods. Future work on carbonates should obtain additional compositional information that will allow robust identification of domains contaminated with extraneous Pb.