During cooling of CO 2 -bearing fluid inclusions the clathrate compound carbon dioxide hydrate (CO 2 .5.75H 2 O) freezes out prior to the freezing of the remaining aqueous solution to ice. When crystallized in aqueous solution, gas hydrates form pure compounds of the encaged species and H 2 O molecules and reject from the hydrate lattice any salts or ions in solution. Thus the residual aqueous solution, after clathration, is more saline than the original solution, and measurement of the salinity using depression of the fusion temperature of ice by salt will give an inaccurate estimate of the salinity of that solution. Although the formation of CO 2 hydrate in CO 2 -bearing fluid inclusions, or any gas hydrates in fluid inclusions, invalidates use of the depression of the fusion temperature of ice for estimating the salinity of the aqueous solution, the depression of the temperature of decomposition of the CO 2 hydrate in the presence of CO 2 liquid and CO 2 gas can be a useful measure of salinity, provided CH 4 and other gases are not present in the inclusion.

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