Oxygen isotope ratios of diagenetically formed minerals in shales and sandstones can provide information about temperatures of diagenesis, mechanisms of clay mineral reactions, degree of openness of the rocks to the movement of water during diagenesis, and sources of water involved in reactions.

The isotopic approach has proven especially effective in studying the shales of the United States Gulf Coast in which the predominant diagenetic reaction involving clays is the conversion of smectite layers to illite layers in mixed-layer illite/smectite. Clay minerals affected by this reaction appear to undergo oxygen isotope equilibration with the ambient water. A byproduct of the smectite-illite transition is quartz. When the diagenetically formed quartz can be isolated for isotopic analysis, oxygen isotope fractionations between coexisting quartz and clay are indicative of diagenetic temperatures when temperatures are higher than 70 or 80°C. There is some evidence that at temperatures above 180°C clay-size detrital quartz may exchange isotopically with pore fluids and ot er rock constituents, perhaps permitting the determination of maximum burial temperatures of shales even in the absence of the smectite-illite conversion reaction.

During diagenesis the shales studied approximate closed systems, the isotopic composition of the diagenetic waters being determined largely by isotopic exchange with the rocks. Water lost form the shale sequences during diagenesis apparently moves outward or upward along cracks, passing out of the system without isotopically affecting overlying shales.

Diagenetic minerals in sandstones, like those of shales, reflect temperatures and isotopic compositions of pore waters. While few isotopic studies of minerals from sandstones have been done to date, those by Land and his coworkers indicate that this is a very promising approach for unraveling and understanding complex histories of diagenetically altered sandstones.

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