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Dolomitization of sediments may be controlled, in part, by their predolomitization diagenetic history. The Pliocene-Pleistocene dolomites on Bonaire underwent a period of minor freshwater diagenesis prior to dolomitization. During this initial stage of diagenesis, some low-magnesian calcite cement formed. Unaltered high-magnesian calcite skeletal fragments were replaced during dolomitization, aragonite was dissolved, and low-magnesian calcite was at first inert and later locally dissolved. The inclusion crystals and crystal molds of calcite cement in dolomite rhombs, the presence of calcite zones in dolomite rhombs, and the preservation of limestone fragments in dolomitized breccias demonstrate that low-magnesian calcite was not replaced during the initial dolomitization. Cloudy centers and clear rims formed when the dolomitizing fluid changed from near saturation with respect to calcite (cloudy centers owing to inclusions and molds) to undersaturation with respect to calcite (inclusion and mold-free rims).
The concentration of Na+ in these dolomites is approximately 350 ppm, and the ^dgr18O values range from +1.97 to +4.1. These data indicate that the dolomitizing fluid was low in Na+ (relative to seawater) but isotopically heavier than most groundwater and, therefore, probably an evaporation-concentrated fresh water.
The data suggest that dolomitization may be climatically controlled. In humid climates, a sediment in the freshwater-seawater mixing zone may undergo rapid calcification owing to the high PCO2 in the groundwater. In arid climates, the water will have a lower PCO2 as a result of limited soil development; therefore, calcification will be slower and chances for dolomitization will be increased.
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