Isotopic U- U- Th- Th analysis using LA-ICPMS for direct U/Th dating of millennium stalagmites

Here we present high sensitivity, in situ Th and U isotope ratio determinations in carbonates using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Online addition of a well characterized 229Th-233U-236U isotopic spike to the laser generated aerosol by means of a desolvating nebulizer enabled to monitor and correct for mass discrimination and elemental fractionation effects of U and Th. The efficacy of inter-element, mass discrimination, and peak tailing baseline corrections were critically evaluated and optimized. Using a “jet” interface ICPMS setup improved the detection efficiency to a yield of 1-2%. Thereby sufficiently high signal intensities have been achieved even for the low abundant isotope 230Th in stalagmites as young as 5,000-year-old with uncertainties (2σM) better than ±200 years. A flowstone sample in secular equilibrium, collected from Northern Calcareous Alps, was analyzed to verify the approach and activity ratios of 1.011 ± 0.066 (2σM) for 230Th/238U were obtained. This approach allows for the reconstruction of accurate age profiles for younger carbonates in particular, and could be applied for the better understanding of past millennium climate variabilities