HR: 1330h
AN: V32C-1024    [PDF]
TI: Laser Ablation MC-ICPMS {\it in situ} Uranium-Series Dating of U-Rich Speleothems.
AU: * Potter, E
EM: potter@erdw.ethz.ch
AF: ETH-Z\"{u}rich, Institute of Isotope Geology and Mineral Resources ETH-Zentrum, NO F51.5 Sonneggstrasse 5, Z\"{u}rich, 8092 Switzerland
AU: Stirling, C H
EM: stirling@erdw.ethz.ch
AF: ETH-Z\"{u}rich, Institute of Isotope Geology and Mineral Resources ETH-Zentrum, NO F51.5 Sonneggstrasse 5, Z\"{u}rich, 8092 Switzerland
AU: Sp\"{o}tl, C
EM: christoph.spoetl@uibk.ac.at
AF: University of Innsbruck, Institute f\"{u}r Geologie und Pal\"{a}ontologie Innrain 52, Innsbruck, 6020 Austria
AU: Halliday, A N
EM: halliday@erdw.ethz.ch
AF: ETH-Z\"{u}rich, Institute of Isotope Geology and Mineral Resources ETH-Zentrum, NO F51.5 Sonneggstrasse 5, Z\"{u}rich, 8092 Switzerland
AB: The timing and nature of rapid climate change events in the paleoclimate record are becoming increasingly under focus. These studies require high-resolution information on a temporal and spatial scale. U-rich speleothems ($\sim$100 ppm) from the Austrian Alps (Sp\"{o}tl {\it et al.}, Geology, 30, 815-818, 2002) record rapid climate transitions, such as glacier advance and retreat during the last several glacial cycles, a time scale that is appropriate for the application of U-series dating. Here we compare conventional U-series solution MC-ICPMS with LA-ICPMS techniques for these samples to demonstrate that {\it in situ} U-series analysis can achieve these goals. In this study, a 193 nm ArF excimer laser has been used for sampling a number of U-rich speleothem samples as well as natural and artificial standards. The ablated material is transported from the ablation cell with a helium gas flow before mixing with argon and subsequent introduction into a Nu Plamsa MC-ICPMS. This instrument is equipped with multiple ion counters at two mass unit separation, a configuration that allows for the simultaneous measurement in a single measurement cycle of the high intensity signals ($^{238}$U and $^{235}$U) with Faraday collectors and the lower intensity signals ($^{234}$U, $^{232}$Th, $^{230}$Th) with ion counters. Ablation analyses, at 120$\mu$m spatial resolution, of samples containing $\sim$100 ppm levels of uranium, consume as little as $\sim$5 ng of $^{238}$U, $\sim$200 fg of $^{234}$U and $\sim$80 fg of $^{230}$Th per analysis and result in within-run precision of up to 1.5$\permil$ for $^{234}$U/$^{238}$U and up to 3$\permil$ for $^{230}$Th/$^{238}$U at the 2$\sigma_M$ level. This is more than a factor of two improvement in precision compared to first generation LA-MC-ICPMS {\it in situ} U-series studies (Stirling {\it et al.}, Geochim. Cosmochim. Acta, 64, 3737-3750, 2000). $^{234}$U/$^{238}$U measurements are highly accurate and reproducible, however the accuracy of $^{230}$Th/$^{238}$U measurements is affected by elemental fractionation between U and Th. In a previous study of the application of LA-MC-ICPMS to in situ U-series analysis Stirling {\it et al.} (2000) noted the effects of elemental fractionation between U and Th and highlighted the need for a matrix matched standard to accurately monitor this fractionation. In this study, we have evaluated several standard candidates. These include both natural secular equilibrium speleothem samples and artificial standards in the form of pressed powder pellets made from crushed speleothem material, and speleothem material that has been dissolved and reprecipitated.
DE: 1035 Geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting