Micromill and in situ laser ablation sampling techniques for high spatial resolution MC-ICPMS U-Th dating of carbonates

Procedures for accurate determinations of 230Th-234U-238U isotope ratios using in situ laser ablation (LA) and micromill techniques for carbonates such as speleothems and corals are presented. For LA analyses we use a New Wave Research UP193HE laser and a new multiple ion counting detector system available for the ThermoFinnigan Neptune MC-ICPMS. Multiple ion counting increases the efficiency of low level ion beam collection by allowing simultaneous collection of all ion beams and also circumvents problems associated with unstable, transient beams. We present results of LA U-series measurements on a U-rich (10–168 µg/g) speleothem from Spannagel Cave (Austria) and of small sub-samples from the same speleothem section prepared using a New Wave Research MicroMill and processed through separation and purification solution chemistry. Using a secular equilibrium speleothem with U concentration of 0.25 µg/g we demonstrate that LA measurements yield accurate results on speleothem samples with less than 0.5 µg/g U. For LA results, we currently obtain 230Th/238U results with precisions of 2% (95% confidence limit) for single isotope ratio measurements on a U-rich sample. On the low U (0.25 µg/g U) sample we achieve a precision between 10 and 20% for single 230Th/238U measurements. Precisions of 1% and 5% (2 σm) can be achieved for high and low U concentration samples, respectively, using 10–15 measurements on coeval sub-samples. 0.4 mg micromill sub-samples of the low U concentration sample yield 230Th/238U isotope ratios with precisions of 2%. Accuracy of LA and micromill measurements is assessed by comparison with MC-ICPMS and TIMS measurements on larger sub-samples. vantages of our LA technique are, for example, high spatial resolution analyses and the possibility of rapid determination of numerous coeval sub-samples. The results on micromill samples are more precise but this sample preparation technique is time consuming and larger sample sizes than used for LA are needed for individual measurements.