Magnesium isotopic analysis of olivine by laser-ablation multi-collector ICP-MS: Composition dependent matrix effects and a comparison of the Earth and Moon

Magnesium isotopic compositions of olivine were measured by laser-ablation multi-collector ICP-MS using an ArF 193 nm excimer laser. Analytical precision based on replicate analyses of mantle olivine is ∼0.2 permil (2SD). This is about an order of magnitude better than has been reported by ion microprobe or thermal ionization mass spectrometry, but about a factor of 2-3 less precise than can be obtained by solution aspiration multi-collector ICP-MS with a similar signal intensity. Analysis of synthetic olivines demonstrates a composition-dependent matrix effect in which systematically heavier isotopic compositions are measured in olivines with lower Mg#. The magnitude of this matrix effect varies with laser operating parameters such as spot diameter and repetition rate, and appears to be due to mass-dependent isotope fractionation at the ablation site rather than within the plasma. After correction for matrix effects, the Mg isotopic composition of olivine from the Moon is shown to be identical with that of the Earth's mantle.