Celia Dalou, Lenny Riguet, Johan Villeneuve, Laurent Tissandier, Thomas Rigaudier, Damien Cividini, Julien Zollinger and Guillaume Paris
Geostandards and Geoanalytical Research, août 2024
Voir en ligne : doi : 10.1111/ggr.12584
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(AESi) compositions (Fe 95 Ni 5 , Fe 90 Ni 10 , Fe 80 Ni 20 , and Fe 80 Ni 15 Si 5) 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 $ 10 6 K s-1. Sulfur contents were determined at the Service d’Analyse des Roches et Minéraux at the CRPG and absolute δ 34 S values were determined by multi-collector ICP-MS (MC-ICP-MS, ThermoScientific Neptune) and isotope ratio mass spectrometry (Thermoscientific Delta V). A δ 34 S value of 16.01 AE 0.31‰ was consistently obtained using the MC-ICP-MS, which was indistinguishable of the δ 34 S value of the FeS starting material (15.95 AE 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 δ 34 S values. However, samples containing > 5000 μg g-1 S showed FeNi-FeS immiscibility, leading to minor dispersion of the S mass fractions and δ 34 S 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. The stable (non-radiogenic) S isotopic compositions of planetary reservoirs are key cosmochemical tracers used to determine the origin(s) of S on Earth. However, discrepancies between the S isotopic compositions of Earth’s rocks (δ 34 S =-1.4 AE 0.5‰; Labidi et al. 2013) compared with those of chondrites (-0.3 AE 0.2‰ in enstatite chondrites and-0.08 AE 0.44‰ to +0.49 AE 0.16‰ in carbonaceous chondrites; Wang et al. 2021) imply that a process fractionated the terrestrial δ 34 S values from those of Earth’s precursors. Among planetary processes that could have fractionated the δ 34 S values of Earth’s mantle, core formation is a likely candidate, given the siderophile (‘iron-loving’) character of S (Tsuno et al. 2018, Li et al. 2016). High-pressure experimental samples are used to quantify elemental and isotopic fractionations during core formation (e.g., Labidi et al. 2016). Bulk measurements are commonly performed to accurately determine isotopic fractionations of between 0.15 and 0.30‰ at the 2s level (Labidi and Cartigny 2016, Labidi et al. 2016). This method, described in Labidi et al. (2012, 2016), requires that metal and silicate phases be hand separated under a binocular microscope to obtain 20-50 mg of silicate and 2-10 mg of metal alloy. Although these amounts are representative of what can be obtained in experimental samples, the typical S abundances in the alloys of Labidi et al. (2016) were between $ 9 and $ 23% m/m, making them iron-1