Analytical strategies for 207 Pb ∕ 235 U carbonate geochronology

Abstract. In carbonate U–Pb geochronology, unknown initial 234U/238U disequilibria can be debilitating for the accuracy of 206Pb/238U dates. Even if residual 234U/238U disequilibrium can be measured precisely enough to perform accurate corrections, beyond ca. 1.5 Ma (depending on the magnitude of initial excess/deficit of 234U) the expansion of uncertainties due to the disequilibrium correction may make carbonate 206Pb/238U dates prohibitively imprecise. An alternative approach utilising the 207Pb/235U system is arguably more accurate, however the lower abundance of 235U and 207Pb relative to 238U and 206Pb inevitably results in larger analytical data-point uncertainties. Here we explore analytical strategies that maximise the potential of the 207Pb/235U 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 208Pb/204Pb, 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 208Pb normalisation imposed by the difficulty of analysing 204Pb may add noise to the results. Overall, precise 207Pb/235U carbonate geochronology is feasible and with minor analytical adjustments it can be adopted broadly instead of the 206Pb/238U method. Future work on carbonates should obtain additional compositional information that will allow robust identification of domains contaminated with extraneous Pb.