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 analytical methodology and theoretical basis to interpret the meaning of fluid inclusions is still under development. We contribute to this field of research by (1) studying natural fluid inclusions in well-constrained geological environments, (2) performing high P–T experiments on synthetic and natural fluid inclusions under controlled conditions, and (3) constructing thermodynamic models to understand the behaviour of fluid inclusions.
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).
Our research contributes to establishing a theoretical framework in which various kinds of fluid inclusions can be interpreted. This involves deriving phase diagrams for isochoric inclusions in model chemical systems and in learning how to extract useful information from inclusions that have not maintained their original composition and volume since their entrapment.