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.
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The past decade has revealed significant advantages to using fluid inclusions as a means of understanding the physical and chemical history of fluids in sedimentary basins, but it also has revealed important limitations which have required that a new approach must be employed to effectively use fluid inclusions. This book is divided into six sections: (1) what fluid inclusions are and what geologic history they are capable of recording; (2) basic phase equilibria that must be known to understand the behavior of pore fluids and fluid inclusions in nature; (3) the question of validity of using fluid inclusions as records of ancient diagenetic systems is dealt with in such a way that the questions commonly asked about the limitations of the technique are addressed; (4) hot to conduct a fluid inclusion study, a new petrographically based approach for conducting fluid inclusion research that is followed by methods that allow for the interpretation of compositions of pore fluids that existed in sedimentary rocks, and methods of geothermometry and geobarometry; (5) selected case histories that are designed specifically to give practice in evaluating fluid inclusion data from the diagenetic realm; and (6) a summary of the arsenal of analytical techniques that may be applied to fluid inclusions to develop additional constraints on fluid inclusion composition.