On cooling during microthermometry, fluid inclusions invariably supercool before freezing under disequilibrium (metastable) conditions to form ice and hydrates. Measurements of fluid inclusions from the Irish Zn-Pb hydrothermal system reveal a strong linear correlation (R2 = 0.968) between the final ice-melting temperature (TmI) and the metastable freezing temperature (Tmf) of the form,  
The relationship is shown to be independent of the heating-freezing stage model, the host mineral, and, largely, inclusion size but is affected by the presence of CO2 and by the cooling rate. The correlation shows that metastable freezing is predictable and, in fact, in small droplets of pure solution, occurs at a well-defined, salinity-dependent temperature, referred to as the homogeneous freezing point. This relationship allows salinity to be estimated in fluid inclusions when the optical recognition of final ice melting is not possible due to small inclusion size or cloudy samples, or when inclusions go into a metastable, vapor-absent state because of the collapse of the bubble on freezing. Using a cooling rate of ~50°C/min, inclusion salinity is given by the following equation:  
Salinity (wt %NaCl equiv)=-69.7-2.617Tmf-0.02603Tmf2-0.0000994Tmf3.

The homogeneous freezing point is controlled by an equilibrium thermodynamic property related to the activity of water. In small droplets of pure solution, as approximated by fluid inclusions, freezing will occur when the water activity is 0.305 above that of the stable ice-melting condition at the same temperature, independent of solute type. “Early” metastable freezing, at a temperature above the homogeneous freezing point, may occur in very large inclusions or in those containing “seed” particles or CO2. In such cases, the salinity will be underestimated by the equation above.

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