Fluid Inclusions in Minerals

Minerals that have grown or have been deformed in the presence of a fluid often trap and enclose tiny aliquots of the fluid. The resulting "fluid inclusions", visible under the microscope in transparent crystals, may preserve the properties (chemical composition, density) of the original bulk paleofluid. As direct samples of paleo-fluids, the study and chemical analysis of fluid inclusions provides invaluable information on past and present rock–water interaction, especially at elevated pressures and temperatures (e.g. in hydrothermal ore deposits, deep aquifers, geothermal systems).

The image above right shows natural fluid inclusions in hydrothermal quartz viewed in transmitted light under the microscope. All the inclusions contain a dark vapour bubble, a halite crystal and a saline aqueous solution. The relative proportions of these phases are the same in all the inclusions, demonstrating that they initially trapped a one-phase saline fluid at high pressure and temperature. During cooling of the quartz from its geological environment to room temperature, the vapour bubbles and the halite crystals exsolved from the trapped fluid in each inclusion (© L.W. Diamond).