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

We have developed a new analytical technique for precise and accurate in situ Hf isotope ratio measurements for zircons by means of laser ablation-multiple collector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS). Newly designed sample cell and N2 mixing technique provided higher elemental sensitivity with smoother signal intensity profiles. In order to improve the accuracy of the Hf isotope ratio analysis, previously reported correction methods for isobaric interferences on 176Hf by 176Yb and 176Lu were rigorously evaluated, and we found that the use of independent mass bias factors for Hf and Yb in the isobaric interference correction resulted in more reliable Hf isotopic data. er 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.