Magnesium isotope analysis of olivine and pyroxene by SIMS: Evaluation of matrix effects

The performance of multi-collector secondary ion mass spectrometry (MC-SIMS) for Mg isotope ratio analysis was evaluated using 17 olivine and 5 pyroxene reference materials (RMs). The Mg isotope composition of these RMs was accurately and precisely determined by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS), and these measured isotope ratios were used to evaluate SIMS instrumental mass bias as a function of the forsterite (Fo) content of olivine. The magnitude of the Mg isotope matrix effects were ~3‰ in δ²⁵Mg, and are a complex function of olivine Fo content, that ranged from Fo_59.3 to Fo₁₀₀. In addition to these Mg isotope matrix effects, Si⁺ ion yields and Mg⁺/Si⁺ ion ratios varied as a complex function of the Fo content of the olivine RMs. For example, Si⁺ ion yields varied by ~33%. Based on the observations, we propose instrumental bias correction procedures for SIMS Mg isotope analysis of olivine using a combination of Mg⁺/Si⁺ ratios and Fo content of olivine. Using this correction method, the accuracy of δ²⁵Mg analyses is 0.3‰, except for analysis of olivine with Fo_{86 – 88} where instrumental biases and Mg⁺/Si⁺ ratios change dramatically with Fo content, making it more difficult to assess the accuracy of Mg isotope ratio measurements by SIMS over this narrow range of Fo content. Five pyroxene RMs (3 orthopyroxenes and 2 clinopyroxenes) show smaller ranges of instrumental bias (~1.4‰ in δ²⁵Mg) as compared to the olivine RMs. The instrumental bias for the 3 orthopyroxene RMs do not define a linear relationship with respect to enstatite (En) content, that ranged from En_{8 5 . 5 – 96.3}. The clinopyroxene RMs have similar En and wollastonite (Wo) contents but have δ²⁵Mg values that differ by 0.5‰ relative to their δ²⁵Mg values determined by MC-ICP-MS. These results indicate that additional factors (e.g., minor element abundances) likely contribute to SIMS instrumental mass fractionation. In order to better correct for these SIMS matrix effects, additional pyroxene RMs with various chemical compositions and known Mg isotope ratios are needed.