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Most sedimentary diagenesis involves recrystalJization or overgrowths on original minerals, or the growth of new phases. This new growth may trap fluid as inclusions that provide data not only on the nature, composition, pressure, and density of the fluids present during diagenesis, but particularly on the temperature at which the host crystals grew. As most optical methods of study require inclusions >1-2 μm in diameter, fine-grained products of diagenesis, in the 10-20 μm range, seldom provide useful material. The possibilities of finding inclusions of useful size increase as the size of the host crystal increases. In spite of this limitation, reasonably valid quantitative or qualitative physical and chemical data, both new and from the literature, have been obtained on inclusions from the following specific diagenetic environments: (1) crystal-lined geodes, vugs, and veins in sediments; (2) Mississippi Valley-type ore deposits; (3) carbonate and quartz cements in detrital rocks; (4) saline and sulfur deposits; (5) petroleum reservoir rocks; (6) sphalerite in bituminous coal beds.

Most inclusion temperatures in these and other similar environments range from 25 to 150° C, and most of the fluids are moderately to strongly saline brines which commonly contain petroleum and as much as tens of atmospheres of methane-rich gas. Homogenization temperatures of inclusions in some Mississippi Valley-type ore deposits are higher than 150° C but seldom more than 200° C. It is concluded that hot, strongly saline fluids have moved through many, if not most, sediments at some time in their history, and that at least part of the diagenetic changes seen have been caused by such fluids.

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