Oxygen Isotope Geothermometry of Diagenetically Altered Shales
The maximum temperature to which a shale has been heated as a result of burial can, in some instances, be estimated using oxygen isotope geothermometry. The isotopic fractionation, or difference between O<sup>18</sup>/O<sup>16</sup> ratios of two coexisting minerals which have reached isotopic equilibrium with one another, is temperature dependent. Hence, if two coexisting minerals, which have isotopically equilibrated with one another, can be separated from a shale, and if the variation of the equilibrium isotopic fractionation between these minerals is known, the temperature of equilibration can-be estimated.
Quartz and coexisting illite or mixed layer illite/smectite is a promising pair for isotope geothermometry of shales. A preliminary equilibrium fractionation curve for this pair is given by:
where exp refers to the fraction of layers in the mixed-layer clay which are expandable.
The results of three isotope geothermometry studies are summarized. Mineralogic and O18/O16 data for coexisting quartz and illite from the altered volcanic rocks of the active hydrothermal region at Broadlands, New Zealand were used to investigate isotopic equilibration and to serve as a basis for calibration of the quartz-clay isotope geothermometer.
Mineralogic and Ol8/Ol6 data for coexisting clay-sized quartz and illite /smectite from one of three deep wells in the Gulf Coast indicate that isotopic equilibrium is approached between these two minerals at a well temperature above about 100° C. The illite/smectite apparently exchanges oxygen with pore waters in an approach toward isotopic equilibrium during the reaction: smectite + Al + K 𠆒 illite + Si. The released Si forms quartz which dominates the finest quartz fractions and forms overgrowths on detrital grains. This quartz apparently forms in isotopic equilibrium with pore waters.
Isotopic temperatures derived from coexisting quartz and illite from the Precambrian Belt argillites range from 225° C to 310° C and generally increase down-section. This temperature range is compatible with the bulk mineralogy and probable depth to which the rocks have been buried.
Oxygen isotope geothermometry can not yet be used routinely. However, it may be more possible to do so after additional information is obtained on factors such as chemical alteration and mineralogic reactions that control isotopic exchange.
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There are a number of gaping holes in accumulated knowledge within the discipline of sedimentology. Perhaps one of the largest holes has been the general subject of diagenesis in clastic rocks. It was therefore fortuitous that two symposia covering various aspects of diagenesis (mainly in clastics) were presented a year apart in different parts of the country but with the same motivation – to contribute to the closing of that knowledge gap. Sedimentologists now have a fairly good idea of the what and the how of sediment deposition. What happens after the sediments are lithified has frequently been ignored. It was the aim of both editors of this publication to approach the subject from two different viewpoints. Schluger directed a symposium which looked mainly at clastic reservoirs, and Scholle presented a symposium which examined various aspects of paleotemperature control of diagenesis.