Are Fluid Inclusions Representative Samples of Diagenetic Fluids?
The phases trapped within fluid inclusions and any phase changes that may subsequently be observed in the lab are dependent not only on the micrometer and submicrometer scale physical and chemical processes active during entrapment, but also on processes that proceed after entrapment. If fluid inclusions are to be useful, it is important to know the degree to which their chemical composition and density are representative of the bulk of the diagenetic fluid from which they were entrapped. The degree to which an inclusion's fluid is representative of the ancient diagenetic fluid involves an assessment of several important questions. First, for any fluid trapped within an inclusion, a question arises as to its similarity with the major, minor, trace element, and isotopic composition of the ancient pore fluid that existed prior to entrapment. Second, if more than one fluid phase were present in a diagenetic system during inclusion formation, a question arises as to whether the phases trapped within inclusions are representative, proportionately and compositionally, to those phases present in the pore fluid. Finally, one must question the potential of natural processes to cause changes in fluid inclusion location, shape, volume, and compositions after initial entrapment and during subsequent uplift and/or burial. This chapter attempts to answer these essential questions in two major parts. First, we will determine the degree to which fluid inclusions (formed from both homogeneous and heterogeneous fluid systems) might differ from the bulk of the diagenetic fluid during the entrapment process. Second, any changes resulting from
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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.