Introduction to Fluid Inclusions and their Applications
For several decades geologists have been applying field, petrographic, and geochemical methods to study the diagenesis of limestones, dolomites, evaporites and sandstones. The most successful studies have integrated field and petrographic work with various geochemical methods. For most applications, the value of any one of the most commonly applied techniques has often been limited; however, when applied together they have proven very useful. Careful petrography has been the most important and reliable component of diagenetic studies. Trace and minor element analysis of diagenetic phases is limited by poor knowledge of distribution coefficients, unknown applicability of distribution coefficients, or unknown pore-fluid chemistry. Interpretation of values of stable isotopes (∂13C and ∂18O) may be plagued by unknown temperature, pore fluid composition, water-rock ratio, or unknown fractionation factors for some systems. All of the foregoing methods are indirect methods of interpreting diagenetic history in that they are the result of diagenetic processes rather than samples of the diagenetic systems themselves. Such indirect approaches often yield data that can easily be misinterpreted.
Fluid inclusions are fluid-filled vacuoles sealed within minerals. When trapped within diagenetic minerals, they provide the only direct means of examining the fluids present in ancient diagenetic environments. Fluid inclusions can be thought of as time capsules storing information about ancient temperatures, pressures, and fluid compositions. They may provide the following valuable information with simple petrographic observation, microthermometric analysis, or sophisticated geochemical analysis of inclusion contents.