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