Long-term reproducibility of multicollector Sr and Nd isotope ratio analysis

Abstract

High-precision multidynamic analyses for Sr and Nd isotopic standards are reported for a 4-year period on the VG354® mass spectrometer at Royal Holloway and Bedford New College (RHBNC). The data are assessed in 8 (Nd) or 11 (Sr) subsets separated by major machine maintenance. Within each subset, results for the Sr standard SRM987 show a consistent correlation between the mean triple-collector multidynamic ${}^{87}\text{Sr}{}^{86}\text{Sr}$ result, normalised for fractionation by the power law, and the mean ${}^{86}\text{Sr}{}^{88}\text{Sr}$ (gradient − 0.020 ± 0.007 2sd). This correlation is perfectly modelled by an exponential fractionation law calculated from exact masses. Both the La Jolla and laboratory Nd standards show consistent correlations in multidynamic data normalised by power law to ${}^{146}\text{Nd}{}^{144}\text{Nd}\text{= 0.7219}$. The mean triple-collector ${}^{143}\text{Nd}{}^{144}\text{Nd}$ vs. mean double-collector ${}^{142}\text{Nd}{}^{144}\text{Nd}$ results show a positive correlation of gradient ± 0.176 ± 0.090 (2sd), while triple-collector ${}^{15}\text{Nd}{}^{144}\text{Nd}$ shows a negative correlation with ${}^{142}\text{Nd}{}^{144}\text{Nd}$, of gradient −0.046 ± 0.040 (2sd). Fractionation-normalisation by exponential law improves the best data a little, but cannot explain the correlation, which may instead relate to minor degeneration of ion optical or collector performance. Exponential correction to the Sr data, and an empirical correction for Nd, result in mean external reproducibility (2sd) of ±0.000019 for ${}^{87}\text{Sr}{}^{86}\text{Sr}$ and ±0.000008 for ${}^{143}\text{Nd}{}^{144}\text{Nd}$. Such reproducibility has important applications in Sr isotope stratigraphy, SmNd geochronology and Nd isotope geochemistry. The best ${}^{142}\text{Nd}{}^{144}\text{Nd}$ data only agree with previously published values if an exponential fractionation correction is applied. This requires the mean present-day chondrite ${}^{143}\text{Nd}{}^{144}\text{Nd}$ to be 0.512646.