High-precision copper and iron isotope analysis of igneous rock standards by MC-ICP-MS

Abstract

Stable isotopic systematics of Cu and Fe are two important tracers for geological and biological processes. Generally, separation of Cu and Fe from a matrix was achieved by two independent, completely different methods. In this study, we report a method for one-step anion-exchange separation of Cu and Fe from a matrix for igneous rocks using strong anion resin AG-MP-1M. Cu and Fe isotopic ratios were measured by multi-collector inductively coupled plasma mass-spectrometry (Neptune plus) using a sample–standard bracketing method. External normalization using Zn to correct for instrumental bias was also adopted for Cu isotopic measurement of some samples. In addition, all parameters that could affect the accuracy and precision of isotopic measurements were examined. Long-term external reproducibility better than ±0.05‰ (2SD) for δ⁶⁵Cu and ±0.049‰ (2SD) for δ⁵⁶Fe was routinely obtained. Cu and Fe isotopic compositions of commercially accessible igneous rock standards including basalt, diabase, amphibolite, andesite and granodiorite were measured using this method. δ⁶⁵Cu values of igneous rock standards vary from −0.01 to +0.39‰ (n = 11) with an overall range (0.40‰) that exceeds about 8 times that of the current analytical precision. The improved precisions of stable Cu isotopic analysis thus demonstrate that igneous rocks are not homogeneous in Cu isotopic composition. The procedure for one-step separation of Cu and Fe and high-precision analysis of Cu and Fe isotopic ratios have an important advantage for economical and efficient study of stable Cu and Fe isotopic systematics in geological and biological fields.