A Plea for More Skepticism Toward Fluid Inclusions: Part I. Postentrapment Changes in Fluid Density and Fluid Salinity Are Very Common

Abstract

Abstract This contribution focuses on fluid inclusions in quartz, which is the most commonly investigated host mineral for fluid inclusions. We demonstrate based on petrographic relationships, cathodoluminescence (CL) images, and microthermometric data that postentrapment migration and/or dismemberment of fluid inclusions in quartz is a very common phenomenon that leads to major changes in the recorded fluid properties. The phenomenon is not restricted to samples that experienced shear stress but is common also in freely grown, euhedral quartz crystals that grew within open-space veins and miarolitic cavities. Generally, both migration and dismemberment lead to substantial increases in fluid density and, thus, in decreased fluid inclusion homogenization temperatures by up to several hundred degrees Celsius. Fluid salinities were commonly less affected, although some high-temperature brine inclusions record salinity increases by up to 30% relative. A good fluid inclusion preservation state is indicated (but not proven) if the inclusions occur on well-defined trails or growth zones, show constant phase proportions, and provide tightly clustered microthermometric data. In contrast, well-developed negative crystal shapes cannot be taken as evidence for a good preservation state. Randomly distributed, regular-shaped fluid inclusions in high-temperature (>300°C) quartz samples are in most cases not of primary origin but rather represent migrated pseudosecondary fluid inclusions. Evidence for fluid inclusion migration and dismemberment includes the occurrence of tiny solid inclusions (mostly mica and rutile) within the host quartz, scattered microthermometric data, and the presence of irregular or c-axis-parallel, dully to nonluminescent quartz patches that depart from healed fracture planes visible in cathodoluminescence images. In summary, pressure-temperature information obtained from fluid inclusions should generally be treated with great caution, unless they meet the above-mentioned criteria for good inclusion preservation.