Fluid inclusions

Fluid inclusions are microscopic bubbles of liquid and gas that are trapped within crystals. As minerals often form from a liquid or aqueous medium, tiny blebs of that liquid can become trapped within the crystal structure or in healed fractures within a crystal. These small inclusions range in size from 0.1 to 1 mm and are usually only visible in detail by microscopic study.

These inclusions occur in a wide variety of environments. For example, they are found within cementing minerals of sedimentary rocks, in gangue minerals such as quartz or calcite in hydrothermal circulation deposits, in fossil amber, and in deep ice cores from the Greenland and Antarctic ice caps. The inclusions can provide information about the conditions existing during the formation of the enclosing mineral.

Hydrothermal ore minerals typically form from high temperature aqueous solutions. The trapped fluid in an inclusion preserves a record of the composition, temperature and pressure of the mineralizing environment. An inclusion often contains two or more phases. If a vapor bubble is present in the inclusion along with a liquid phase, simple heating of the inclusion to the point of resorption of the vapor bubble gives a likely temperature of the original fluid. If minute crystals are present in the inclusion, such as halite, sylvite, hematite, or sulfides are present, they provide direct clues as to the composition of the original fluid.

In the recent years, fluid inclusion research has been extensively applied to understand the role of fluids in the deep crust and crust-mantle interface. Fluid inclusions trapped within granulite facies rocks have provided important clues on the petrogenesis of dry granulite facies rocks through the influx of CO₂-rich fluids from sub-lithospheric sources. CO₂-rich fluid inclusions were also recorded from a number of ultrahigh-temperature granulite facies terranes suggesting the involvement of CO₂ in extreme crustal metamorphism. Some recent studies speculate that CO₂ derived by sub-solidus decarbonation reactions during extreme metamorphism has contributed to the deglaciation of the snowball Earth (Santosh and Omori, 2008).

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