High precision determinations of 87Rb/85Rb in geologic materials by MC-ICP-MS

In comparison to thermal ionization mass spectrometry (TIMS), multiple collector-inductively coupled plasma mass spectrometry (MC-ICP-MS) is better suited to obtain precise measurements of isotope ratios for elements that have only two stable isotopes such as Rb. The instrumental mass bias of Rb is corrected with admixed Zr following Waight et al. [T. Waight, J. Baker, B. Willigers, Chem. Geol. 186 (1-2) (2002) 99]. The technique presented here uses the 92Zr/90Zr for mass bias correction of the 87Rb/85Rb on the Micromass IsoProbe MC-ICP-MS. For an assumed 92Zr/90Zr value of 0.33339, a 87Rb/85Rb of 0.38554 ± 30 (2 S.D.) is obtained for the NBS-984 Rb standard solution over the course of 1 year. This value agrees with the presently accepted IUPAC ratio obtained from TIMS measurements. Within single analytical sessions, the reproducibility of 87Rb/85Rb measurements is 0.02-0.05% (2 S.D.). Consequently, this technique provides up to a 10-fold improvement in precision over conventional TIMS techniques, potentially enabling the detection of isotope fractionation effects that possibly occur during geologic and biologic processes. In addition, the technique is applicable for high precision Rb-Sr geochronology, potentially yielding ages with an uncertainty of ±0.1% (2 S.D.).