An integrated approach for quantifying fluid inclusion data combining microthermometry, LA-ICP-MS, and thermodynamic modeling

Abstract

The development of LA-ICP-MS and other microanalytical techniques for in situ analysis of single fluid inclusions has made it possible to quantify the elemental concentration of major elements, trace metals, sulfur and halogens in fluid inclusions from a wide range of geological environments. The established quantification methods typically interpret the LA-ICP-MS and microthermometric data (melting points of ice, salt and hydrate phases) in terms of simple binary or ternary model systems (e.g., H2O-NaCl and H2O-NaCl-CaCl2), which can be inaccurate for complex natural geofluids such as multi-solute basinal brines, magmatic-hydrothermal fluids or pegmatitic fluids. Another approach uses mass or charge balance constraints along with the elemental ratios obtained from LA-ICP-MS. This improves the quantification of more complex fluids to some extent, but because it is based on the equivalent concentration of NaCl from the NaCl-H2O binary system, it typically introduces systematic uncertainty in the concentrations of all elements analyzed. These limitations can be fundamentally overcome by combining microthermometric and LA-ICP-MS data with thermodynamic modeling. In this approach, elemental ratios (element/Na) are directly calculated from the LA-ICP-MS data and then the unknown Na concentration is calculated by modeling the chemical equilibrium between the aqueous solution and the last melting solid phase (e.g., ice, hydrohalite, sylvite) of the fluid inclusion using a Pitzer model for the aqueous solution. Comparison between modeled and experimental data of chloride systems (including salt-solution equilibria and synthetic fluid inclusion data) demonstrates excellent agreement over wide ranges of temperature and composition. Our new model, accompanied by a new approach for quantifying uncertainty, is more general than previous methods and makes it possible to quantify complex multi-element fluid inclusions from diverse geological environments including magmatic, metamorphic, sedimentary and various ore-forming hydrothermal fluids.