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

Rationale: Measurements of the multiple sulfur isotopic composition (delta S-34, delta S-33 and delta S-36 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 (Delta S-33 and Delta S-36 values). Here we propose some improvements to the earlier published femtosecond laser-ablation fluorination method to make it suitable for measuring both Delta S-33 and Delta S-36 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 SF6 gas from other fluorinated products produced by fluorination of sulfide minerals. Compared with the previous version of the purification system, the efficiency of SF6 purification was significantly improved, which in turn improved the precision of the method for in situ GC/IRMS measurements of S-36/S-32 ratios (and determination of Delta S-36 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 similar to 12-13 nmol SF6 (optimal size), the overall precision of the method is +/- 0.03 parts per thousand for Delta S-33 values and +/- 0.27 parts per thousand for Delta S-36 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 delta S-34, Delta S-33 and Delta S-36 values in sulfide minerals.