INTEGRATED U-PB AGE AND HF ISOTOPES IN ZIRCON: A POWERFUL TOOL FOR PETROGENETIC STUDIES

The Hf isotopic composition of zircon can provide powerful constraints on the petrogenenis of igneous and metamorphic rocks, especially on the source materials from which a given sample suite is derived. Indeed, combined U-Pb age + Hf isotope data are now commonly produced in nearly all zircon-based studies. However, the utility of Hf isotopes can only be realized when accurate ages can be assigned to Hf isotopic compositions. While this is typically not a problem for simple zircon populations, it can be problematic for 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.

In order to overcome this potential uncertainty, simultaneous analysis of the same zircon volume is required, and is best achieved using the laser ablation “split stream” (LASS) technique where the aerosol from laser ablation is split between two ICP mass spectrometers for simultaneous analysis of U-Pb and Hf isotope compositions (e.g., Yuan et al., 2004, Chemical Geology). We typically us a 30-40 micron spot and 60, 1-second measurements for Hf isotopes and 300 sweeps from ²⁰²Hg to ²³⁸U for U-Pb. Using this approach, we demonstrate the application of this technique to a range of petrogenetic problems. Examples will be presented which demonstrate that 1) concurrent measurement of U-Pb data along with Hf isotopes can be used as a filter to produce unambiguous Hf isotopic composition in complex zircons and 2) that the combination of U-Pb and Hf isotopes is a powerful technique for determining the nature of zircon growth and alteration in zircon populations.