High precision zircon SIMS Zr isotope analysis

Abstract

Zirconium isotope is a very promising tracer to investigate geological processes such as magmatic crystallization and differentiation, and to tackle the secular evolution of the continental crust. With zircon being its major hosting mineral, developing analytical technique with high accuracy, high precision and high efficiency is critical to facilitate its geological applications. This study presents an analytical method to measure zircon Zr isotope compositions by secondary ion mass spectrometry (SIMS). With careful sample preparation to limit the topography effect and instrumental tuning for maintaining its stability, we demonstrate that large geometry SIMS can provide zircon Zr isotope measurements with high accuracy, high precision and high efficiency. Based on extensive testing on our CAMECA IMS 1280HR, the recommended instrumental tuning parameters are: a primary beam intensity of ∼30 nA, a beam diameter of ∼20 × 15 μm², a mass resolution power of 2400 and a raster size of 15 × 15 μm², with ⁹⁴Zr and ⁹⁰Zr being collected by static Faraday cups via a signal accumulation time of 20 cycles × 4 s. By analyzing zircon reference materials commonly used for U–Pb and O analysis (i.e., 91500, Penglai, Plešovice, and Mudtank), we have achieved a δ⁹⁴Zr repeatability of 0.04–0.07‰ (2SD, n = 33–75), and the Zr isotope compositions of zircon reference materials measured by SIMS agree remarkably well with those defined by double-spike solution methods. Our method consumes ∼1.2 ng materials in a single spot analysis (∼0.9 μm pit depth), together with its high spatial resolution (∼13 × 14 × 0.3 μm³ for data collection) and high efficiency (∼4 minutes per analysis), it is expected that our SIMS Zr isotope analytical method will be very useful for studying precious materials (e.g., zircon in lunar soil) and to facilitate big data research (e.g., detrital zircon study).