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524650 
Journal Article 
Measurement of SIMS Instrumental Mass Fractionation of Pb Isotopes During Zircon Dating 
Stern, RA; Bodorkos, S; Kamo, SL; Hickman, AH; Corfu, F 
2009 
33 
2 
145-168 
English 
An igneous zircon reference material (OG1) was characterised for U-Pb isotopes by ID-TIMS, and utilised to evaluate SIMS (SHRIMP) instrumental mass fractionation (IMF) of radiogenic Pb isotopes (Pb-207*/Pb-206*). The TIMS Pb-207*/Pb-206* reference value for OG1 was 0.29907 +/- 0.00011 (95% confidence limit), 3465.4 +/- 0.6 Ma. The high Pb-207* (similar to 30 mu g g(-1)), negligible common Pb, and isotopic homogeneity permitted precise (+/- 1-2 parts per thousand) Pb-207*/Pb-206* measurements within the analytical sessions. External reproducibility of mean Pb-207*/Pb-206* ratios between sessions was demonstrated for one instrument, yielding a mean IMF of +0.87 +/- 0.49 parts per thousand. The mean Pb-207*/Pb-206* ratios between instruments were dispersed beyond uncertainties, with session IMF values from +3.6 +/- 1.7 parts per thousand to -2.4 +/- 1.3 parts per thousand, and a grand mean IMF value (twenty-six sessions) of +0.70 +/- 0.52 parts per thousand, indicating a tendency towards elevated Pb-207*/Pb-206*. The specific causes of variability in IMF are unclear, but generally reflect subtle differences in analytical conditions. The common practice in SIMS of assuming that IMF for Pb+ is insignificant could result in systematic age biases and underestimated uncertainties, of critical importance for precise correlation of Precambrian events. Nevertheless, a zircon RM such as OG1 can be readily incorporated into routine dating to improve Pb-207*/Pb-206* accuracy and external reproducibility. 
Pb isotopes; zircon; instrumental mass fractionation; reference; material; U-Pb geochronology; ion microprobe; isotopes de Pb; fractionnement de masse instrumental; matEriau de rEfErence; gEochronologie U-Pb; microsonde ionique; ion microprobe; age-determinations; id-tims; geochronology; lead; spectrometry; precision; samples