HF ANALYSIS IN ZIRCON BY LA-MC-ICPMS: PROMISE AND PITFALLS

The analysis of Hf isotopes in zircon by LA-MC-ICPMS is a relatively new analytical technique that is being used increasingly as a monitor of magmatic processes. The advantage of this system is integration of the crystallization history of the zircon from the U-Pb chronometer with its Hf isotope composition, which can provide important information on the sources that contributed to form the zircon. This integrated information can be used in a wide range of applications including igneous petrogenesis, identifying inherited components in magmatic sources, and helping constrain models of crustal evolution. How these data are best interpreted, and the questions they can be used to address, however, are related to the total uncertainty in the analyses. Sources of potential uncertainty include: 1) uncertainty during the Hf analysis (in-run uncertainty); 2) uncertainty in the correction of the interference of ¹⁷⁶Yb and ¹⁷⁶Lu on ¹⁷⁶Hf; 3) matrix effects between zircon grains; 4) instrumental biases in the determination of Hf isotopic composition (e.g., normalization to the accepted value of ¹⁷⁶Hf/¹⁷⁷Hf).

Using only in-run uncertainty for individual zircon Hf analyses underestimates the potential sources of uncertainty. One solution in assessing overall uncertainty in a laser Hf measurement is to analyze a suite of zircon standards with Lu/Hf and Yb/Hf ratios that closely match the unknown zircons being analyzed, using the same parameters as used for the unknowns. We have determined the Lu-Hf isotopic compositions on solutions of several well-known zircon standards purified by ion chromatography for use in assessing the quality of laser Hf analyses in our laboratory: R33, ¹⁷⁶Hf/¹⁷⁷Hf = 0.282767 ± 18, ¹⁷⁶Lu/¹⁷⁷Hf = 0.00177 (n=4); Temora, ¹⁷⁶Hf/¹⁷⁷Hf = 0.282672 ± 12, ¹⁷⁶Lu/¹⁷⁷Hf = 0.00106 (n=3); FC1, ¹⁷⁶Hf/¹⁷⁷Hf = 0.282182 ± 14, ¹⁷⁶Lu/¹⁷⁷Hf = 0.00101 (n=5); QGNG, ¹⁷⁶Hf/¹⁷⁷Hf = 0.281607 ± 9, ¹⁷⁶Lu/¹⁷⁷Hf = 0.00078 (n=4); and 91500, ¹⁷⁶Hf/¹⁷⁷Hf = 0.282311 ± 6, ¹⁷⁶Lu/¹⁷⁷Hf = 0.00030. The variance of laser analyses of these zircon standards relative to their solution values provides a fair and reasonable estimate of the overall uncertainty of individual analyses of unknowns.

This presentation will give examples of how this technique has been applied effectively and others where its application may be potentially problematic.