SPLIT-STREAM ZIRCON U-PB AGE AND HF-ISOTOPE LASER ABLATION ANALYSIS: A POWERFUL TECHNIQUE IN THE DETRITAL ZIRCON ANALYTICAL TOOLBOX (Invited Presentation)

The so-called “split stream” analysis describes a technique where the aerosol from laser ablation is split between two ICP mass spectrometers for simultaneous analysis of the ablated material (e.g., Yuan et al., 2004, Chem. Geol.). One of the most useful applications of this technique is simultaneously analysis of zircon for U-Pb age and Hf isotope composition. The obvious advantage of this technique is in determining age and isotopic composition on the same zircon volume. This is especially important in the analysis of complexly zoned zircons with inherited cores and/or different domains of zircon growth. Most commonly, Hf isotopes are analyzed in zircons previously analyzed for U-Pb age by either ICPMS or ion microprobe (e.g., SHRIMP). In the case of the former, care is taken to analyze the same zircon growth zone for Hf isotopic composition, but because zircon spot analyses are chosen based on their 2-D cathodoluminescence images, the domain excavated at depth may not be the same as imaged at the surface, and two separate volumes are analyzed. In the case of prior ion microprobe age analysis, the volume of the ion microprobe pit will be less than 1% of the laser Hf analysis and also represents a very different zircon volume. Thus, in zircons with either age or isotopic complexity, analysis of different volumes presents a large source of potential uncertainty. Simultaneous analysis of the same zircon volume removes this uncertainty. Further, simultaneous determination of age and Hf composition improves sampling efficiency, important in detrital zircons studies where large numbers of analyses are required.

Our instrument set up at WSU involves a New Wave UP-213 Nd-YAG laser with the resulting aerosol delivered to a ThermoFinnigan Element2 high-resolution ICPMS for U-Pb age analysis and a ThermoFinnigan Neptune multi-collector ICPMS for Hf isotope analysis. Our typical configuration consists of a 40-micron spot and 60, 1-second measurements for Hf isotopes and 300 sweeps over the mass range ²⁰²Hg to ²³⁸U for U-Pb. Using this approach, the precision and accuracy of Hf isotope analyses are nearly the same as for Hf alone. There is some fall-off in the quality of the U-Pb analyses but still quite sufficient to determine age heterogeneity. We will present data that demonstrates the utility of the split-stream approach.