Matrix-independent Fe isotope ratio determination in silicates using UV femtosecond laser ablation

UV femtosecond laser ablation coupled to MC-ICP-MS provides a promising in situ tool to investigate elemental and isotope ratios by non-matrix-matched calibration. In this study, we investigate Fe isotope composition in siliceous matrices including biotite, hornblende, garnet, fayalite and forsterite (San Carlos Olivine), and an oceanic Fe–Mn crust using the iron reference material IRMM-014 for calibration. To test the accuracy of the laser ablation data, Fe isotope compositions were obtained independently by solution ICP-MS after chromatographic separation of Fe. Sample materials with low Cr content, i.e. biotite, hornblende, fayalite and the Fe–Mn crust, reveal δ56/54Fe and δ57/54Fe values that agree with those from solution ICP-MS data within the measured precision. For high Cr concentration (54Cr/54Fe >0.0001), i.e. in the garnet and forsterite sample, δ56/54Fe and δ57/54Fe values were derived from 57Fe/56Fe ratios as correction of the isobaric interference of 54Cr on 54Fe is unsatisfactory. This approach provides accurate results for both minerals. Moreover, the garnet crystal exhibits isotopic zonation with differences of 0.3‰ in δ56/54Fe showing that substantial heterogeneities exist in high-temperature metamorphic minerals. Multiple analyses of homogeneous sample materials reveal a repeatability of 0.1‰ (2 SD) for δ56/54Fe and 0.2‰ (2 SD) for δ57/54Fe, respectively. This study adds to the observations of Horn et al. (2006) who have shown that the determination of Fe isotope ratios in various matrices including iron alloys, iron oxides and hydroxides, iron sulfide and iron carbonates can be performed with high accuracy and precision at high spatial resolution using UV femtosecond laser ablation ICP-MS. These results demonstrate that femtosecond laser ablation ICP-MS is a largely matrix-independent method, which provides a substantial advantage over commonly employed nanosecond laser ablation systems.