High‐precision in situ silicon isotopic analyses by multi‐collector secondary ion mass spectrometry in olivine and low‐calcium pyroxene

Abstract : Rationale: High-precision determination of silicon isotopes can be achieved by in situ multi-collector secondary ion mass spectrometry. The analyses accuracy is however sensitive to ion yields and instrumental mass fractionations (IMFs) induced by the analytical procedure. These effects vary from an instrument to another, with the analytical settings, and with the composition and nature of the sample. Because ion yields and IMF effects are not predictable and rely on empirical calibrations, high-accuracy analyses require suitable sets of standards. Methods: Here, we document calibrations of ion yields and matrix effects in a set of 23 olivine standards and 3 low-Ca pyroxene for silicon isotopic measurements in both polarities using Caméca IMS 1270 E7 and IMS 1280 HR2 ion probes set with the cesium or radiofrequency (RF) source. Results: Silicon ion yields show (i) strong variations with the chemical composition, and (ii) an opposite behavior between the secondary positive and negative polarities. The magnitude of IMF along the fayalite-forsterite (olivine) series shows a complex behavior, increasing overall by ≈7‰ (secondary positive) and ≈15‰ (secondary negative) with increasing olivine Mg#. A drastic change in olivine IMF occurs at Mg# ≈ 70 in both polarities. The magnitude of IMF for low-Ca pyroxene from Mg# = 70-100 is almost constant in both polarities, i.e. ≈0.1‰ in secondary positive and ≈0.15‰ in secondary negative. Analytical uncertainties on individual analyses were ±0.05–0.15‰ (2 S.E.) with both sources, and external errors for each standard material were ≈ ±0.05–0.5‰ (2 S.E.) with the Cs source and ≈ ±0.03–0.15‰ (2 S.E.) with the RF source. Conclusions: The IMF effect of Si isotopes in silicates shows complex behaviors that vary with the chemistry and the settings of the instrument. We developed a suitable set of standards in order to perform high-accuracy in situ measurements of Si isotopes in olivine and low-Ca pyroxene characterized by varying chemical compositions by MC-SIMS.
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Johan Villeneuve, Marc Chaussidon, Yves Marrocchi, Zhengbin Deng, E. Bruce Watson. High‐precision in situ silicon isotopic analyses by multi‐collector secondary ion mass spectrometry in olivine and low‐calcium pyroxene. Rapid Communications in Mass Spectrometry, Wiley, 2019, 33 (20), pp.1589-1597. ⟨10.1002/rcm.8508⟩. ⟨hal-02357497⟩

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