Fe Isotope Variations in Natural Materials Measured Using High Mass Resolution Multiple Collector ICPMS

We present the first measurements of Fe isotope variations in chemically purified natural samples using high mass resolution multiple-collector inductively coupled plasma source mass spectrometry (MC-ICPMS). High mass resolution allows polyatomic interferences at Fe masses to be resolved (especially, ⁴⁰Ar¹⁴N⁺, ⁴⁰Ar ¹⁶O⁺, and ⁴⁰Ar¹⁶OH⁺). Simultaneous detection of Fe isotope ion beams using multiple Faraday collectors facilitates high-precision isotope ratio measurements. Fe in basalt and paleosol samples was extracted and purified using a simple, single-stage anion chemistry procedure. A Cu "element spike" was used as an internal standard to correct for variations in mass bias. Using this procedure, we obtained data with an external precision of 0.03-0.11‰ and 0.04-0.15‰ for δ^56/54Fe and δ^57/54Fe, respectively (2σ). Use of Cu was necessary for such reproducibility, presumably because of subtle effects of residual sample matrix on mass bias. These findings demonstrate the utility of high-resolution MC-ICPMS for high-precision Fe isotope analysis in geologic and other natural materials. They also highlight the importance of internal monitoring of mass bias, particularly when using routine methods for Fe extraction and purification.