High-precision zirconium stable isotope measurements of geological reference materials as measured by double-spike MC-ICPMS
Résumé
Zirconium plays a major role in geochemistry as it is the major cation of zircons - the oldest preserved minerals on Earth. While Zr isotopic anomalies in meteorites have been widely studied, mass dependant Zr stable isotope fractionation during geological processing has been untouched. Here, we report Zr stable isotopic data for terrestrial igneous rocks and present a novel method for the determination of Zr stable isotope ratios within natural geological materials using ion exchange, double-spike, multiple-collector inductively coupled mass spectrometry (MC-ICPMS). Zirconium is isolated from the rock matrix via a chromatographic separation protocol using a first pass column with AG1-X8 anion exchange resin, and a second pass column containing Eichrom (R) DGA resin. A Zr-91-Zr-96 double-spike was created from enriched single Zr-91 and Zr-96 isotope spikes. Samples were combined with the Zr double-spike at a 43: 57 spike: sample [Zr] ratio, prior to dissolution and column chemistry. After column chemistry the purified sample solutions were analysed on a Thermo Scientific Neptune Plus MC-ICPMS and the data was reduced using IsoSpike, with the final Zr isotope data being reported as the per mil deviation of the Zr-94/Zr-90 from the IPGP-Zr standard (delta Zr-94/90(IPGP-Zr)). The delta Zr-94/90(IPGP-Zr) of six igneous standard reference materials: two basalts (BHVO-2 and JB-2), one andesite (AGV-2), two granites (GA and GS-N) and a serpentinite (UB-N) as well as one individual zircon grain (Plesovice zircon), are presented using this method. Sample measurements are presented with an analytical uncertainty of +/- similar to 0.05% (2sd) for delta Zr-94/90(IPGP-Zr) and these rocks exhibit isotopic variations of similar to 0.15% for delta Zr-94/90(IPGP-Zr). These results demonstrate that natural variations of Zr isotopes occur within terrestrial igneous rocks, and are resolvable with this method. Finally the variation of delta Zr-94/90(IPGP-Zr) values observed within the magmatic rock reference materials is correlated (R-2 = 0.78; n = 5) with SiO2, suggesting that Zr isotopes could serve as a sensitive tracer of magmatic processes.