Hromatographic separation and multicollection-ICPMS analysis of iron. Investigating mass-dependent and -independent isotope effects

Abstract

A procedure was developed that allows precise determination of Fe isotopic composition. Purification of Fe was achieved by ion chromatography on AG1-X8 strongly basic anion-exchange resin. No isotopic fractionation is associated with column chemistry within 0.02‰/amu at 2σ. The isotopic composition was measured with a Micromass IsoProbe multicollection inductively coupled plasma hexapole mass spectrometer. The Fe isotopic composition of the Orgueil C11 carbonaceous chondrite, which best approximates the solar composition, is indistinguishible from that of IRMM-014 (-0.005 ± 0.017‰/amu). The IRMM-014 reference material is therefore used for normalization of the isotopic ratios. The protocol for analyzing mass-dependent variations is validated by measuring geostandards (IF-G, DTS-2, BCR-2, AGV-2) and heavily fractionated Fe left after vacuum evaporation of molten wüstite (FeO) and solar (MgO-Al<inf>2</inf>O<inf>3</inf>-SiO<inf>2</inf>-CaO-FeO in chondritic proportions) compositions. It is shown that the isotopic composition of Fe during evaporation of FeO follows a Rayleigh distillation with a fractionation factor α equal to (m<inf>1</inf>/m<inf>2</inf>) ^1/2, where m<inf>1</inf> and m<inf>2</inf> are the masses of the considered isotopes. This agrees with earlier measurements and theoretical expectations. The isotopic composition of Fe left after vacuum evaporation of solar composition also follows a Rayleigh distillation but with a fractionation factor (1.013 22 ± 0.000 67 for the ⁵⁶Fe/⁵⁴Fe ratio) that is lower than the square root of the masses (1.018 35). The protocol for analyzing mass-independent variations is validated by measuring terrestrial rocks that are not expected to show departure from mass-dependent fractionation. After internal normalization of the ⁵⁷Fe/ ⁵⁴Fe ratio, the isotopic composition of Fe can be measured accurately with precisions of 0.2ε and 0.5ε at 2σ for ⁵⁶Fe/ ⁵⁴Fe and ⁵⁸Fe/⁵⁴Fe ratios, respectively (ε refers to relative variations in parts per 10 000). For ⁵⁸Fe, this precision is an order of magnitude better than what had been achieved before. The method is applied to rocks that could potentially exhibit mass-independent effects, meteorites and Archaean terrestrial samples. The isotopic composition of a 3.8-Ga-old banded iron formation from Isua (IF-G, Greenland), and quartz-pyroxene rocks from Akilia and Innersuartuut (GR91-26 and SM/GR/171770, Greenland) are normal within uncertainties. Similarly, the Orgueil (CI1), Allende (CV3.2), Eagle Station (ESPAL), Brenham (MGPAL), and Old Woman (IIAB) meteorites do not show any mass-independent effect.