Precise determination of the lutetium isotopic composition in rocks and minerals using multicollector ICPMS

Evidence of ¹⁷⁶Hf excess in select meteorites older than 4556Ma was suggested to be caused by excitation of long-lived natural radionuclide ¹⁷⁶Lu to its short-lived isomer ^176mLu, due to an irradiation event during accretion in the early solar system. A result of this process would be a deficit in ¹⁷⁶Lu in irradiated samples by between 1‰ and 7‰. Previous measurements of the Lu isotope ratio in rock samples have not been of sufficient precision to resolve such a phenomenon. We present a new analytical technique designed to measure the ¹⁷⁶Lu/ ¹⁷⁵Lu isotope ratio in rock samples to a precision of ∼0.1‰ using a multicollector inductively coupled mass spectrometer (MC-ICPMS). To account for mass bias we normalized all unknowns to Ames Lu. To correct for any drift and instability associated with mass bias, all standards and samples are doped with W metal and normalized to the nominal W isotopic composition. Any instability in the mass bias is then corrected by characterizing the relationship between the fractionation factor of Lu and W, which is calculated at the start of every analytical session. After correction for isobaric interferences, in particular ¹⁷⁶Yb, we were able to measure ¹⁷⁶Lu/¹⁷⁵Lu ratios in samples to a precision of ∼0.1‰. However, these terrestrial standards were fractionated from Ames Lu by an average of 1.22 ± 0.09‰. This offset in ¹⁷⁶Lu/¹⁷⁵Lu is probably caused by isotopic fractionation of Lu during industrial processing of the Ames Lu standard. To allow more straightforward data comparison we propose the use of NIST3130a as a bracketing standard in future studies. Relative to NIST3130a, the terrestrial standards have a final weighted mean δ¹⁷⁶Lu value of 0.11 ± 0.09‰. All samples have uncertainties of better than 0.11‰; hence, our technique is fully capable of resolving any differences in δ¹⁷⁶Lu of greater than 1‰.