High-Resolution In Situ Fe Isotope Measurements of Silicate Minerals and Glasses by Femtosecond Laser Ablation MC-ICP-MS

Abstract

In this study, we present a high precision and high spatial resolution in situ Fe isotope protocol using femtosecond (fs) laser ablation multi-collector inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS). The intermediate measurement precision obtained over a period of ca. 3 years for the USGS glass BIR-1G against the Puratronic reference material is 0.17‰ (2s) for δ⁵⁶Fe. Uncertainties achieved on individual analyses of glass and olivines were < 0.15‰ for δ⁵⁶Fe. This high precision is associated with high spatial resolution of about 170 × 25 μm. Our results display good consistency between LA-MC-ICP-MS and solution nebulisation MC-ICP-MS data from the literature. Obtained δ⁵⁶Fe values on different USGS glasses (BIR-1G, BHVO-2G and BCR-2G) show that these reference materials have homogenous Fe isotope ratio and therefore can be used as bracketing calibrators during laser ablation measurement sessions. On the other hand, the San Carlos Olivine displays high Fe isotope heterogeneity, and therefore cannot be considered as a good bracketing standard (calibrator). We also applied our fs-LA-MC-ICP-MS protocol to olivines and pyroxene from the Kerguelen Archipelago. This technique appears to be a relevant tool to resolve isotopic zoning in chemically zoned silicate phenocrysts, even of small size (< 1 mm). We demonstrate that within single lavas, olivine crystals display various zoning depending on their size, related to their residence time in the magma. Both equilibrium and diffusive processes were observed in olivine crystals from Kerguelen, uncovering complex histories. Hence, iron isotope ratio measurements by fs-LA-MC-ICP-MS open new possibilities for studying highly zoned silicate minerals in magmatic rocks to better understand their formation.