Improved Cl–Br–I analysis using LA-ICP-MS/MS and new constraints on the source of ligand-rich metamorphic fluids in the Mount Isa Province, Australia

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Halogens play a major role in ore formation as they are important ligands for transporting metals. Determining the halogen content of minerals can provide insights into the composition of ore-forming fluids, but presents significant analytical challenges. Halogen analysis by inductively coupled plasma mass spectrometry (ICP-MS) is hampered by high ionisation energies and significant interferences, including molecular species and doubly charged ions, which contribute to elevated background levels and low signal-to-background ratios. ICP-MS/MS instruments (also referred to as triple quadrupole (QQQ) instruments) have great potential for improving in-situ halogen analysis by laser ablation through the reaction and removal of these interferences. Experiments using H 2 as a reaction gas within the ICP-MS/MS collision-reaction cell demonstrated increased sensitivity and signal-to-background ratios for Cl and Br. Helium as a collision gas within the ICP-MS/MS collision-reaction cell significantly improved the detection of Cl, enabling accurate measurements down to 60 μg/g. Addition of N 2 (~3 mL/min) to the carrier gas further increased the signal-to-background ratios for all analysed elements. While laser ablation-induced background signals are an inherent limitation in halogen analysis, our results demonstrate that their impact can be effectively mitigated through an optimised analytical strategy, with the implementation of the S-lens configuration playing a central role in the observed improvements in sensitivity and overall analytical performance. The developed method and data processing approach enable accurate measurement of Br concentrations down to 2 μg/g for an 85 μm spot size. Iodine analysis is complicated by laser ablation-induced background I signals considered to represent I remobilised from the laser sample cell and/or introduction system. The paucity of low-level I reference materials limits the determination of an experimentally constrained limit of quantification for I and hence a conservative limit of quantification of 0.9 μg/g is defined for an 85 μm spot size. Application of the new approach to metamorphic scapolite from the Mount Isa Province, Australia, demonstrates that the scapolites contain high I/Cl ratios of 5–35 (× 10 6 ) and low Br/Cl ratios of 0.04–0.51 (× 10 3 ). These halogen signatures are consistent with an evaporitic protolith for the scapolite-bearing rocks, with the halogens derived from dissolution of halite with variable contributions from I-rich pore fluids that had interacted with organic matter.