1 THERMODYNAMIC CALCULATION OF SOLUTE CONCENTRATIONS IN FLUID INCLUSIONS BASED ON ELEMENT RATIOS AND MICROTHERMOMETRIC DATA

Recent improvements in chemical analysis of fluid inclusions, using techniques such as LA-ICP-MS, PIXE, SXRF and LIBS for individual inclusions, and crush–leach analysis for bulk samples), now permit concentration ratios of solute elements to be determined with high accuracy. In order to apply these results to geochemical problems of hydrothermal processes, the element ratios must be converted to absolute concentrations in the inclusions. Approaches to this conversion problem have remained very approximate so far, and have not kept pace with the improved quality of the raw analytical data. A strategy to obtain the absolute concentrations is to combine the element ratios with an indicator of the colligative properties of the analysed fluid inclusions, such as the melting temperature of solid daughter phases (e.g. ice, hydrohalite, halite, gas-hydrate clathrates, etc.) [1,2]. Melting temperatures can be measured by routine microthermometry to an accuracy of better than 0.2°C, thus providing a sensitive measure of bulk salinity. Traditionally in the fluid inclusion literature, the observed depression of melting points is expressed in terms of equivalent NaCl concentration. However, it is widely recognised that the presence of solutes other than NaCl can strongly influence the melting temperatures of daughter crystals, and hence influence estimates of bulk salinity. Previous methods to correct for these effects [1,2,3] have been restricted to very simple chemical systems (ternary o