Mobility and H2O loss from fluid inclusions in natural quartz crystals

Petrographic observations on quartz crystals from the Mole Granite (Austral ia) and other localities shed new light on the mechanisms of post-entrapmen t modification of fluid inclusions. These modifications include migration a way from pseudosecondary trails, changes in fluid,salinity and density, sha pe distortion and the formation of "sweat-haloes" around strongly deformed inclusions. Increases in fluid salinity, which usually are associated with inclusion migration, indicate water-losses of up to 50%. However, LA-ICP-MS -analysis of unmobilized and mobilized inclusions of the same trail reveals basically unchanged ratios of major - and trace element cations, with the exception of Li, which seems to be incorporated into the crystal lattice du ring migration. Despite the fact that all these modifications are closely r elated to deformation processes, they occur not only in mechanically deform ed quartz, but also in free-standing crystals. In the latter samples. stres s has been generated internally as a result of brazil-twinned growth and co mpositional zonation. These observations and their interpretation leads to a list of practical criteria that should help in differentiating between re liable and suspect fluid inclusions in other samples.