Precise Measurement of Magnesium Isotopes in Fe-Mg Minerals Using a Multi-collector SHRIMP Ion Microprobe

Abstract

The distribution of Mg isotopes in minerals is becoming increasingly relevant in Earth science. Usually, they are determined by dissolving mineral concentrates and, after purifying Mg with ion exchange resins, analysing the resulting solutions by TIMS or, most often, MC-ICP-MS. When applied to individual minerals, these methods are slow and prone to contamination from impurities in the concentrates, inconveniences that may be avoided using spot analysis techniques such as LA-MC-ICP-MS or SIMS, albeit at the price of a large instrumental mass fractionation (IMF) and isobaric interferences, most prominent in the former. Here, we studied the potential of the multi-collector SHRIMP II ion microprobe for measuring Mg isotopes in Fe-Mg silicates and oxides. We found that, when corrected for the divergence of the Mg ion paths within the sample chamber caused by the Earth’s magnetic field, the SHRIMP’s IMF overwhelmingly depends on the mineral species, and the effects of variable chemical composition are negligible. We propose that the IMF is caused by the force constant difference, ΔF, between "hard" and "soft" bonds linking the ions of the studied element to the mineral lattice. Given that ΔF is a constant for each mineral species, we calculated IMF-correction factors for the most common Mg-bearing minerals. The thus-calculated correction factors permit the analysis in the same session, and with reasonable accuracy (within ~0.3‰ of the δ26Mg determined by SN-MC-ICP-MS analyses of concentrates), of samples from different mineral species, facilitating the application of Mg isotopes to terrestrial studies.