Fluid inclusions with gas bubbles as geothermometers

Abstract

Two types of fluid inclusions can be distinguished. The first is based on the assumption of Sorby (1858) that a homogeneous phase, such as water, salt solution, or CO2, is entrapped, meaning that the bubbles result from the gas of the enclosed fluid. The second type includes foreign gas entrapped with the fluid. “Sorby”-type inclusions can be used as thermometers if either the formation pressure is known, or the pressure impact has been shown to be unimportant. It is only possible to neglect pressure in cases where the degree of filling is high enough that bubbles vanish at low temperatures. If, during entrapment, the fluid included dissolved foreign gas that was released during cooling, it is even more dangerous to equate filling with formation temperatures. Compared with Sorby-type inclusions, even less information is available about the expected large-pressure impact. If foreign gas was entrapped as bubbles, the filling temperature may significantly deviate from the formation temperature and may even increase at first during heating. Such non-Sorby-type inclusions can be identified by measuring the degree of filling and comparing the related filling temperature with that of water and C02, respectively, at a similar degree of filling. The question of the composition of the inclusions is important not only for determining temperatures, but also for solving questions about the formation and alteration of rocks. Attention is drawn to the method of Brewster (1826), who determined the refractive index by using the total reflection. All these remarks are valid for both primary and secondary inclusions. The decrepitation method is not an appropriate means to distinguish primary from secondary inclusions. Further, it is not suited to determine the type of inclusion, or the degree of filling.