A Plea for More Skepticism Toward Fluid Inclusions: Part II. Homogenization via Halite Dissolution in Brine Inclusions from Magmatic-Hydrothermal Systems Is Commonly the Result of Postentrapment Modifications

Abstract

Abstract Quartz-hosted brine inclusions that homogenize via halite dissolution occur in many magmatic-hydrothermal systems. In many cases their calculated minimum entrapment pressures are geologically unrealistically high, which has been interpreted to reflect accidental entrapment of halite crystals, and hence, to indicate halite saturation. Here it is demonstrated based on microthermometric trends and laser ablation-inductively coupled plasma-mass spectrometry analyses of fluid inclusion assemblages from eight different ore deposits that the phenomenon of inclusion homogenization via halite dissolution is commonly caused by postentrapment changes in fluid inclusion volume rather than by cotrapping of halite. Many of the investigated assemblages show highly variable liquid-vapor homogenization temperatures at relatively constant fluid salinity—a trend that cannot be produced by cotrapping of halite or by cotrapping of vapor. Furthermore, in those assemblages that do display variable fluid salinities, the elemental Cl/X concentration ratios (where X denotes any element other than Na) do not increase with increasing fluid salinity. The observed microthermometric and compositional trends are best explained by variable extents of postentrapment reduction in inclusion volume, which increased fluid density and in some cases was accompanied by H2O loss and migration of the inclusions within the host quartz. The fact that such trends can be observed even in seemingly well preserved fluid inclusion assemblages suggests that postentrapment modifications in fluid inclusion volume and H2O contents may be widespread and thus that temperature and pressure estimates from quartz-hosted fluid inclusions should generally be treated with caution. This is particularly true for samples that may have experienced deformation, such as massive quartz veins and rock-forming quartz.