Improvements of precision and accuracy in in situ Hf isotope microanalysis of zircon using the laser ablation-MC-ICPMS technique

In order to test the reliability of the Hf isotopic data from ∼35 μm diameter ablation pits that are smaller than the typical size of most zircon grains, Lu-Hf isotopes for three zircons (Harvard 91500, Antarctic zircon PMA7 and Acasta zircon AY199) were measured. The present 176Hf / 177Hf isotope ratio of 0.282321 ± 46 (2 S.D.) and initial 176Hf / 177Hf isotope ratio of 0.282315 ± 47 (2 S.D.) were obtained for zircon standard 91500. The resulting present 176Hf / 177Hf isotope ratio shows an excellent agreement with the previously reported data. Our in situ isotopic data revealed that the Lu / Hf ratio varied significantly within the 91500 grain (176Lu / 177Hf = 0.000130-0.000345), suggesting the existence of slight isotopic heterogeneity in present 176Hf / 177Hf (approximately 15 ppm). For Antarctic zircon PMA7, despite the smaller size of the single grain (50-100 μm wide, 80-300 μm long) and the presence of various mineral inclusions, resulting analytical precision for the initial 176Hf / 177Hf ratio was 128 ppm [0.281208 ± 36 (2 S.D.)]. For Acasta zircon AY199, despite the high Yb / Hf (∼0.05) and Lu / Hf (∼0.01) isotope ratios with the early Archean crystallization age (3.74 Ga), resulting analytical precision for the initial 176Hf / 177Hf ratio was 187 ppm [0.280160 ± 52 (2 S.D.)]. These results demonstrate clearly that in situ Hf isotope ratio microanalysis developed here has the potential to become a significant tool for zircon Lu-Hf isotopic study.