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Measurement of SIMS Instrumental Mass Fractionation of Pb Isotopes During Zircon Dating
Author(s) -
Stern Richard A.,
Bodorkos Simon,
Kamo Sandra L.,
Hickman Arthur H.,
Corfu Fernando
Publication year - 2009
Publication title -
geostandards and geoanalytical research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.037
H-Index - 73
eISSN - 1751-908X
pISSN - 1639-4488
DOI - 10.1111/j.1751-908x.2009.00023.x
Subject(s) - zircon , radiogenic nuclide , isotope , fractionation , geology , precambrian , reproducibility , analytical chemistry (journal) , isotope fractionation , mineralogy , geochemistry , chemistry , environmental chemistry , mantle (geology) , physics , nuclear physics , organic chemistry , chromatography
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 ( 207 Pb*/ 206 Pb*). The TIMS 207 Pb*/ 206 Pb* reference value for OG1 was 0.29907 ± 0.00011 (95% confidence limit), 3465.4 ± 0.6 Ma. The high 207 Pb* (∼ 30 μg g −1 ), negligible common Pb, and isotopic homogeneity permitted precise (± 1–2‰) 207 Pb*/ 206 Pb* measurements within the analytical sessions. External reproducibility of mean 207 Pb*/ 206 Pb* ratios between sessions was demonstrated for one instrument, yielding a mean IMF of +0.87 ± 0.49‰. The mean 207 Pb*/ 206 Pb* ratios between instruments were dispersed beyond uncertainties, with session IMF values from +3.6 ± 1.7‰ to −2.4 ± 1.3‰, and a grand mean IMF value (twenty‐six sessions) of +0.70 ± 0.52‰, indicating a tendency towards elevated 207 Pb*/ 206 Pb*. 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 207 Pb*/ 206 Pb* accuracy and external reproducibility.