An improved femtosecond laser‐ablation fluorination method for measurements of sulfur isotopic anomalies (∆ ³³ S and ∆ ³⁶ S) in sulfides with high precision

Abstract

Rationale Measurements of the multiple sulfur isotopic composition (δ 34 S, δ 33 S and δ 36 S values) of ancient sedimentary sulfide are useful for clarifying and reconstructing the picture of the global sulfur cycle on the early Earth. The methods used for these measurements should provide a high level of precision for the determination of sulfur isotope mass‐independent anomalies (∆ 33 S and ∆ 36 S values). Here we propose some improvements to the earlier published femtosecond laser‐ablation fluorination method to make it suitable for measuring both ∆ 33 S and ∆ 36 S values in the Archean sedimentary sulfides with acceptable precision. Methods A new gas purification system for the laser‐ablation fluorination method has been developed. The design of this system is based on temperature‐controlled flow traps for cryogenic separation of SF 6 gas from other fluorinated products produced by fluorination of sulfide minerals. Compared with the previous version of the purification system, the efficiency of SF 6 purification was significantly improved, which in turn improved the precision of the method for in situ GC/IRMS measurements of 36 S/ 32 S ratios (and determination of ∆ 36 S values). Results The improved method was tested using IAEA reference materials, as well as samples of natural pyrite with zero and non‐zero sulfur isotope anomalies. For samples of ~12–13 nmol SF 6 (optimal size), the overall precision of the method is ±0.03‰ for ∆ 33 S values and ±0.27‰ for ∆ 36 S values. This level of precision is satisfactory to examine the sulfur isotope anomalies in the rock. Conclusions The improved femtosecond laser‐ablation fluorination method is applicable for in situ measurements of δ 34 S, ∆ 33 S and ∆ 36 S values in sulfide minerals.