Synthesis and Characterization of Metallic (Fe-Ni, Fe-Ni-Si) Reference Materials for SIMS 34S/32S Measurements

Abstract

Secondary ion mass spectrometry (SIMS) is often used to determine the sulfur contents and isotope ratios of metallic alloys in meteorites or high-pressure experimental samples. However, SIMS analyses involve calibration and the determination of instrumental mass fractionation in reference materials with a matrix composition similar to that of the unknown samples. To provide metallic reference materials adapted to S measurements via SIMS, we synthesised a series of twenty-eight alloys comprising four FeNi(±Si) compositions (Fe₉₅Ni₅, Fe₉₀Ni₁₀, Fe₈₀Ni₂₀, and Fe₈₀Ni₁₅Si₅) with S contents varying from 100 μg g⁻¹ to 4 g/100g using the “melt spinning” method, which guarantees that the metal alloys are rapidly quenched at ~ 10⁶ K s⁻¹. Sulfur contents were determined at the Service d'Analyse des Roches et Minéraux at the CRPG and absolute δ³⁴S values were determined by multi-collector ICP-MS (MC-ICP-MS, ThermoScientific Neptune) and isotope ratio mass spectrometry (Thermoscientific Delta V). A δ³⁴S value of 16.01 ± 0.31‰ was consistently obtained using the MC-ICP-MS, which was indistinguishable of the δ³⁴S value of the FeS starting material (15.95 ± 0.08‰). It suggests that S did not undergo isotopic fractionation during the melting process. Of fifteen samples containing ≤ 5000 μg g⁻¹ S, SIMS measurements with 15-μm-diameter spots were repeatable to within 10% relative (1 standard deviation, 1s) for S contents and 2‰ for δ³⁴S values. However, samples containing > 5000 μg g⁻¹ S showed FeNi–FeS immiscibility, leading to minor dispersion of the S mass fractions and δ³⁴S values. No matrix effect was observed for Fe-Ni, Si, or S contents in terms of the calibration curves and instrumental mass fractionation. We ultimately recommend eight samples as reliable reference materials for S isotopic measurements by SIMS, which we can share worldwide with other laboratories.