Exploring the advantages and limitations of in situ U-Pb carbonate geochronology using speleothems

Abstract

The recent development of methods for in situ U-Pb age determination in carbonates has found widespread application, but the benefits and limitations of the method over bulk analysis (isotope dilution - ID) approaches have yet to be fully explored. Here we use speleothems - cave carbonates such as stalagmites and flowstones - to investigate the utility of in situ dating methodologies for "challenging"matrices with typically low U and Pb contents and predominantly late Cenozoic ages. Using samples for which ID data have already been published, we show that accurate ages can be obtained for many speleothem types by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Consideration of our own and literature data suggest that most carbonates with >1ĝppm uranium and a few hundred parts per billion of Pb should be good targets for in situ methodologies, regardless of age. In situ analysis often provides a larger spread in U/Pb ratios, which can be advantageous for isochron construction, but isochron ages rarely achieve the ultimate precision of ID analyses conducted on the same samples simply because signal sizes are dramatically reduced. LA analysis is faster than ID and thus will play a significant role in reconnaissance studies. The major advantage of the in situ methodology appears to be the potential for successful dating outcomes in sample types requiring high spatial-resolution analysis or those with a high common-Pb component where LA approaches may facilitate identification of the most radiogenic regions for analysis.