The occurrence of clathrates (solid gas hydrates) in fluid inclusions severely complicates the use of microthermometry to estimate the composition and density of aqueo-carbonic fluids. Although models are available to predict clathrate phase equilibria, the models are based on chemical engineering data from ideal systems. Microthermometric and laser Raman microspectrometric studies of both natural and synthetic aqueo-carbonic inclusions provide evidence that in the presence of clathrates such inclusions are not at equilibrium. Contrary to predictive models, two or more separate clathrates of distinct composition can occur within an inclusion, in local equilibrium with either the aqueous solution or the carbonic phase rather than the bulk fluid composition. These results suggest a problem with the use of predictive thermodynamic models based on final clathrate melting temperatures and the assumption of equilibrium within aqueo-carbonic fluid inclusions. Data are presented which demonstrate that, as a result of the formation of clathrate, differences in the degree of aqueous fill can result in very different microthermometric behaviour for inclusions of similar carbonic composition and density. Although the solubility of CO2 in water is low, it increases rapidly with decreasing temperature. This increase can affect both the CO2 content and the density of the carbonic phase, and may invalidate the use of ThCO2 at low temperatures, in the metastable absence of clathrate, to calculate carbonic density.
