Abstract

Cathodoluminescence (CL), trace element concentrations, and isotope compositions of luminescently zoned poikilitic and mosaic calcite cements of the upper Smackover Formation are studied in the Harmony, Pachuta Creek, East Nancy, Goodwater, and Garland Creek fields in Clarke County, Mississippi. These calcites precipitated during burial in a depth range of 100 m to 3 km, a temperature range of 30-100 degrees C, and a time span of 60 my. The calcium carbonate for calcite cementation was primarily derived locally by pressure dissolution of the Smackover Formation. Luminescent patterns are consistent in all samples within each field. However, calcites from individual fields differ in number of zones and CL intensity. There is no compelling evidence that equivalent zones in separate fields precipitated simultaneously. It is therefore unwise to correlate individual zones or groups of zones as time horizons from one field to another. Magnesium content of all calcites ranges from about 500-1000 ppm in the oldest zones and increases to about 3000-4000 ppm toward the younger zones. The low Mg (super 2+) content of the older zones is the result of precipitation from waters with low Mg (super 2+) /Ca (super 2+) ratio, typical of subsurface waters. The increase in Mg (super 2+) toward the younger zones may have resulted from precipitation over time at progressively higher temperatures and the increase in D (sub Mg 2 ) . from a pore water whose Mg (super 2+) /Ca (super 2+) ratio was declining. Strontium varies from 100-300 ppm and does not exhibit systematic variation across zones but increases with increasing Mn (super 2+) content. Low Sr (super 2+) content of the calcites is the result of very slow precipitation at high temperatures from a water with low Sr (super 2+) /Ca (super 2+) ratio. Co-variation of Sr (super 2+) with Mn (super 2+) may reflect preferential Sr (super 2+) substitution for Ca (super 2+) in order to reduce lattice distortion caused by the substitution of Mn (super 2+) for Ca (super 2+) . Non-luminescing to dull zones contain less than 30 ppm Mn (super 2+) , while bright zones have 300-1500 ppm Mn (super 2+) . Iron is nearly absent in all zones regardless of CL intensity. Low Mn (super 2+) content of older zones is attributed to calcite precipitation in a reduced environment where Mn (super 2+) was not available. The increase in Mn (super 2+) content in younger zones is the result of importation of Mn (super 2+) through local conduits from the underlying Norphlet Formation, coupled with an increase in D (sub Mn 2 ) due to a progressive increase in temperature during burial. Absence of Fe (super 2+) in all zones may have been the result of precipitation of iron-sulfides when Fe (super 2+) - and Mn (super 2+) -bearing waters came into contact with H 2 S generated by organic maturation of the lower Smackover Formation. delta 18 O decreases and 87 Sr/ 86 Sr becomes more radiogenic toward the younger zones, while delta 13 C remains constant. The lower delta 18 O and more radiogenic 87 Sr/ 86 Sr composition of the younger zones suggest calcite precipitation at higher temperature and influx of radiogenic strontium from the underlying Norphlet Formation. The absence of any CL quenchers, lack of any CL activators other than Mn (super 2+) , and geologic and geochemical considerations suggest that CL intensities were primarily controlled by the Mn (super 2+) /Ca (super 2+) ratio of pore waters and temperature.

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