Reassessment of the NBS SRM-607 K-feldspar as a high precision Rb/Sr and Sr isotope reference

We report high precision Rb/Sr and Sr isotope analyses for the NBS SRM-607 standard feldspar, which is widely used as a reference material for Rb–Sr ratios as well as Sr isotope compositions. The Rb–Sr ratios of different aliquots of the standard were obtained by isotope dilution using MC-ICPMS for Rb isotope analyses with admixed Zr for mass bias correction and TIMS for Sr isotope measurements. Different ∼ 10 mg sample aliquots show variations in Rb/Sr (± 0.74%) and Sr isotopes (± 0.33%) that are outside the certified values and far outside analytical uncertainties. Age related correlation of Rb/Sr with Sr isotope composition indicates that sample heterogeneity is a major reason for large inter-laboratory differences reported for this standard. Nevertheless, the standard can be used as a high precision reference for Rb–Sr calibration, if Rb/Sr and Sr isotope composition are referenced to a Rb–Sr model isochron. In this case the uncertainty on the Rb/Sr of the standard is ± 0.24%.