Combined LA-ICP-MS microanalysis of iodine, bromine and chlorine in fluid inclusions

Abstract

Halogens are excellent tracers of hydrothermal fluid sources in sedimentary, magmatic, metamorphic and ore-forming environments. In situ LA-ICP-MS analysis of Cl and Br in single fluid inclusions has provided fundamentally new insight into the processes leading to the formation of economic mineral deposits. Enrichment and depletion of Br relative to the dominant halogen Cl may be caused by several processes besides seawater evaporation and halite dissolution, which cannot be unequivocally resolved on the basis of Br/Cl ratios alone. Triple-halogen analysis (Cl, Br and I) would provide the required additional constraint permitting discrimination between possible alternative processes. We present the first iodine concentration data of fluid inclusions determined by LA-ICP-MS microanalysis, using the natural scapolite reference material Sca17 for external standardization. The robustness of the method is verified by LA-ICP-MS analysis of synthetic fluid inclusions, which were prepared at 600 °C, and 100 or 200 MPa in cold-seal pressure vessels from four starting solutions containing Na, Rb, Cs, Cl, Br and I. LA-ICP-MS analysis yielded average I concentrations in excellent agreement with the starting solutions. Average concentrations in inclusions synthesized from the solution with 27 μg g⁻¹ I were 27.3 ± 3.9 μg g⁻¹ I (14.3% RSD), and 77.6 ± 6.4 μg g⁻¹ I (8.3% RSD) for the solution with 78 μg g⁻¹ I. Chlorine and Br reference concentrations were reproduced to better than 10% and 20% RSD, respectively. The method was subsequently applied to a pilot study of natural fluid inclusion samples from three geological settings (Alpine metamorphic quartz veins, post-Variscan quartz-Pb–Zn veins, and calcite veins with Mn oxide and hematite mineralization). The data demonstrate significant differences between the samples, which can be interpreted in terms of Br and I enrichment due to reactions with organic matter and fluid mixing processes between evaporite brines and halite dissolution brines. This highlights the potential of triple-halogen LA-ICP-MS analysis in fluid inclusions for tracing the source of crustal fluids in a wide range of geological environments.