Systematics of stretching of fluid inclusions; I. Fluorite and sphalerite at 1 atmosphere confining pressure

More than 1,300 measurements on fluid inclusions in fluorite and sphalerite indicate that stretching proceeds systematically and predictably. In order to generate internal pressures that are sufficiently high to cause stretching, most inclusions must be heated beyond their initial homogenization temperatures, i.e., overheated. The amount of overheating necessary to initiate stretching depends on the P-V-T-X properties of the inclusion fluid, the inclusion size and shape, physical properties of the host mineral, and the confining pressure. In the range of homogenization temperatures and salinities examined in this study, the amount of overheating necessary to initiate stretching of inclusions in fluorite is inversely related to inclusion volume. The results of this study show that the measured homogenization temperature of a fluid inclusion may be considerably higher than the true homogenization temperature if the internal pressure reached sufficiently high values during previous testing in the laboratory or, less likely, during postentrapment thermal events in nature. The systematic relationship between the internal pressure necessary to initiate stretching and the inclusion volume provides a means of recognizing previously stretched inclusions and estimating the magnitude of postentrapment thermal events. Furthermore, reproducibility of homogenization temperature measurements, lack of microscopically observable fractures in the inclusion walls, and the lack of a noticeable increase in the vapor bubble size after heating are insufficient to prove that an inclusion has not stretched.