Abstract
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 >1 ppm 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.
Highlights
The U–Pb decay scheme has played a key role in the chronology of carbonate rocks for more than 3 decades (e.g. Moorbath et al, 1987; Jahn and Cuvellier, 1994; Rasbury and Cole, 2009) utilizing predominantly isotope dilution (ID; i.e. bulk sample) methods
In order to assess the accuracy of the method for relatively low-concentration samples we have analysed a number of speleothems for which we have already produced and published ages by solution ID methods (Tables 2 and 3); these display a wide range of radiogenic : common-Pb ratios but, in all cases, have U in the low parts per million range and Pb in the low parts per billion range, typical of many speleothems
In all of these cases the ID data were obtained using a 233U–205Pb isotopic tracer calibrated against EarthTime reference solutions, and accuracy was constantly monitored by reference to EarthTime synthetic zircon solutions run concurrently
Summary
The U–Pb decay scheme has played a key role in the chronology of carbonate rocks for more than 3 decades (e.g. Moorbath et al, 1987; Jahn and Cuvellier, 1994; Rasbury and Cole, 2009) utilizing predominantly isotope dilution (ID; i.e. bulk sample) methods. Moorbath et al, 1987; Jahn and Cuvellier, 1994; Rasbury and Cole, 2009) utilizing predominantly isotope dilution (ID; i.e. bulk sample) methods. Still in its infancy, this method has already been applied to the chronology of marine cements (Li et al, 2014), vein calcites associated with faulting (Roberts and Walker, 2016; Hansman et al, 2018; Parrish et al, 2018), and the alteration of oceanic crust (Coogan et al, 2016). A thorough exploration of the utility of in situ techniques to speleothem (secondary cave calcite such as stalagmites and flowstones) research has not been conducted, U–Pb dating of speleothems is widely used in studies of climate change (e.g. Vaks et al, 2013; Sniderman et al, 2016), human evolution and migration Speleothems offer a variety of unique analytical challenges for in situ analysis – not least because of their highly variable and often very low levels of radiogenic Pb, and because of the fact that most samples of interest are relatively young – predominantly Neogene or early Quaternary.
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