Systematic evaluation of the effects of post-entrapment modifications to fluid inclusion properties and implications to data interpretation in the H2O-NaCl system

Fluid inclusion studies typically report temperature, pressure and fluid composition data of hydrothermal systems. The inclusions follow specific evolution paths after their entrapment. Post-entrapment modifications will alter the paths to form fluid inclusions with a different homogenization behavior if phase boundaries are crossed. Low-salinity fluid inclusions may cross the locus of critical points due to post-entrapment processes, forming assemblages with variable bubble sizes and different homogenization behavior, while their homogenization temperature may only slightly change. Homogenization temperature, pressure, and homogenization behavior of high-salinity inclusions are generally changed to a larger extent by volume contraction/expansion than this is the case for low-salinity inclusions. Water loss commonly accompanies volume changes of the host mineral by the hydrolytic weakening process. Therefore, the lowest salinity inclusion should have lost the least water in a fluid inclusion assemblage (FIA).

The linear relationship between salinity and homogenization temperature in FIAs from many porphyry deposits can be produced by the combination of volume contraction and water loss, and the variable slopes rely on the relative contribution of each process. The hydrolytic weakening of quartz is prone to occur at relatively high temperature conditions, therefore, there is a characteristically increasing trend in the variation of homogenization temperature with temperature for individual FIAs. Decreased homogenization temperature in the early-trapped high-temperature fluid inclusions will lead to under-estimation of the trapping temperature and hinder the understanding of timing and mechanism of metal precipitation in magmatic-hydrothermal deposits.

Post-entrapment modifications of fluid inclusions may significantly change the homogenization behavior and the temperature-pressure-composition data, obstructing the reconstruction of ore-forming conditions, metamorphic processes, basin fluid evolution, etc. We comprehensively reviewed the effects of post-entrapment modifications to fluid inclusion properties by numerical calculations and reevaluated published fluid inclusion data from many hydrothermal deposits.