Determination of the composition and molar volume of H2O-CO2 fluid inclusions by microthermometry

H2O-CO2 fluid inclusions are common in nature but their accurate analysis b y microthermometry has been hindered by insufficient characterisation of th e fluid properties along the solvus (high temperature miscibility boundary) of the system. Thus some of the previous methods of calculating fluid incl usion composition (x) and molar volume (V) have relied on notoriously unrel iable optical estimates of phase volume fractions. Although the T-V-x prope rties of the solvus can be modelled thermodynamically from equations of sta te, a review shows that none of the published equations are accurate enough to be useful in fluid inclusion microthermometry. Therefore, the available experimental data have been used to construct a new V-x diagram, contoured for phase transition temperatures and phase volume fractions. This diagram allows the bulk composition and molar volume of individual fluid inclusion s to be determined accurately from two microthermometric measurements: the temperature of partial homogenisation of the carbonic phases (in the presen ce of excess aqueous liquid), and the temperature of total homogenisation. Copyright (C) 2000 Elsevier Science Ltd.