Determination of the zirconium isotopic composition of the new isotopic standard NRC ZIRC-1 using MC-ICP-MS

Abstract

High precision analytical methods for ‘non-traditional’ stable isotopes rely upon standards for normalizing isotopic ratios, correcting instrumental mass fractionation, and comparing data between different laboratories. Zirconium isotope measurements are a new and rapidly expanding field, displaying potential for tracing the history of magmatic differentiation, as well as the formation and evolution of crustal reservoirs. However, previous studies have reported Zr isotopic data relative to different Zr standards, with the IPGP-Zr standard being most commonly used. This situation hinders direct inter-laboratory comparison thereby enlarging uncertainties when converting the data. Here the Zr isotopic composition of a new, commercially available isotopic standard ZIRC-1 is systematically calibrated relative to IPGP-Zr through a collaborative approach between two different laboratories using sample-standard bracketing and double spike methods. Moreover, the Zr isotopic composition of the certified reference materials BHVO-2 and AGV-2, alongside the widely studied Allende chondrite, is reported to ensure the robustness of our data and allow for future inter-laboratory data comparison. Consistent results are obtained from the two laboratories and the average δ⁹⁴Zr_{IPGP_Zr} (the permil deviation of the ⁹⁴Zr/⁹⁰Zr of the sample from the IPGP-Zr standard) of ZIRC-1 is −0.220 ± 0.006‰ (2SD, n = 7). The δ⁹¹Zr_{IPGP_Zr}, δ⁹²Zr_{IPGP_Zr}, δ⁹⁴Zr_{IPGP_Zr} and δ⁹⁶Zr_{IPGP_Zr} values of ZIRC-1 follow a mass-dependent relationship. The δ⁹⁴Zr_{IPGP_Zr} and δ⁹⁴Zr_ZIRC-1 values of the CV3 carbonaceous chondrite Allende are −0.041 ± 0.021‰ and 0.179 ± 0.022‰ (2SD, n = 7), respectively, and lower than those of bulk Earth, which has δ⁹⁴Zr_{IPGP_Zr} and δ⁹⁴Zr_ZIRC-1 values of 0.041 ± 0.041‰ and 0.261 ± 0.041‰ respectively, according to the reference of Tian et al. (2021). The BHVO-2 and AGV-2 aliquots analyzed along with ZIRC-1 show agreement with literature data, demonstrating that the data from this study are accurate. The renormalized delta values of all other Zr isotopic standards are listed, allowing for further inter-laboratory comparison.