Experimental decrepitation and re-equilibration of fluid inclusions in synthetic quartz

Abstract

Abstract Strong discrepancies between densities of the fluids trapped in fluid inclusions (F.I.) and P-Tp couples deduced from other geological considerations are common. So, it was decided to test experimentally the possible F.I. re-equilibrations when bringing the host crystal in P-Tp conditions, different from trapping conditions. Runs were conducted on synthetic quartz in a gas apparatus and in a Griggs solid medium apparatus. The following modifications of the F.I. were observed: 1. (1) drastic shape changes of the primary F.I., and appearance of clusters or trails of small secondary inclusions sometimes related to a crack 2. (2) lowering of the fluid densities. Such a re-equilibration is a decrepitation phenomenon: first, hydraulic fracturing, then sealing of the cracks with appearance of secondary inclusions. When using the gas apparatus, the new measured densities are in very good accordance with the nominal P-Tp conditions. When using the solid-medium apparatus, the new densities are surprisingly low relative to the nominal P-T conditions, probably due to a strongly anisotropic stress state in the apparatus.