FLUID INCLUSIONS 259 volatiles, such as CH4 and N2, will lower this melting temperature Similarly, the melting point of pure water is lowered by the addition of salts, such as NaCl, KCl, and CaCl2, in solution By measuring this depression and referring to published freezing point depression curves, it is possible to estimate the salt content or salinity of aqueous inclusions Mostly, this is performed with reference to the simple NaCl H2O system, and the results are usually expressed as equivalent weight per cent NaCl to take into account the fact that components other than NaCl are also usually present In practice, because of supercooling effects, the melting temperatures of frozen inclusions, rather than their freezing temperatures, are used to estimate salinities Figure Principle of the isochore intersection method applied to coexisting oil and aqueous inclusions The bubble point curve can be calculated from estimated oil compositions (e.g., from gas chromatography analysis of the inclusions) The homogenization temperature of the oil, Th(oil), can be used to calculate the P T trapping path of the oil inclusion in the single phase region above this curve The point at which the isochore intersects the homogenization temperature line for coexisting aqueous inclu sions, Th(Aq), defines the P T trapping conditions See Munz In: Andersen T, Frezotti M L, and Burke EAJ (eds.) (2001) Petroleum inclusions in sedimentary basins: systematics, analytical methods and applications Lithos 55: 185 212 The Th values of melt inclusions can be used effectively as independent geobarometers in cases in which silicate melts and associated fluid inclusions have been trapped coevally The isochores for these associated aqueous or gaseous inclusions can be used to fix the pressure or depth of formation Chemical Compositions of Geological Fluids from the Analysis of Inclusions A wide variety of methods are available to determine the chemical compositions of fluid and melt inclusions Simple optical methods Simple optical methods are often very effective For example, the Becke line test can be used to distinguish between aqueous fluid and silicate glass phases because of their contrasting refractive indices Ultraviolet (UV) fluorescence microscopy can be used to identify petroleum inclusions, and the identification of daughter minerals can sometimes be based on their optical and crystallographic properties Microthermometry Microscope heating and freezing stage studies (microthermometry) are frequently used to identify CO2 and associated volatiles and to estimate salinities of aqueous inclusions Pure CO2 should melt at À56.6 C (the triple point) Other Bulk methods A range of instrumental methods with the required sensitivity (parts per million to parts per billion range) are available for the analysis of inclusions in bulk samples (100 mg to 1000 g) Gas chromatography and mass spectrometry are widely used to determine volatile and hydrocarbon contents and the stable isotopic compositions of inclusion fluids released by crushing or heating The analysis of cations and anions in aqueous leachates from crushed samples has often been used to determine the chemical compositions of aqueous inclusions However, in common with other bulk methods, there may be significant problems of contamination from the host or admixed minerals, and it is not always possible to obtain a large enough sample with a single or dominant generation of inclusions Point methods Various spectroscopic and microbeam methods have been applied to the in situ analysis of fluid and melt inclusions These include electron microprobe analysis, which is particularly suited to the analysis of melt inclusions, and protoninduced X-ray emission (PIXE) and synchrotron X-ray fluorescence (SXRF) spectroscopy, which are particularly suited to the analysis of trace elements and ore metals in melt and brine inclusions However, the methods which have had the largest impact in recent years are laser Raman microprobe analysis (LRM) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) LRM is nondestructive and is widely used to identify the solid, liquid, and volatile components in unopened inclusions down to a few micrometres in size It is particularly useful for determining the molar volumes of CO2, N2, CH4, and other Raman-active volatiles in aqueous inclusions and H2O in silicate melt inclusions It has also been used to identify a wide variety