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One of the oldest problems in sedimentology is the origin of dolomite. Dolomite, CaMg (CO3)2, is one of the most common sedimentary carbonate minerals. Its density is 2.866, and calcite's density is 2.710. The absolute abundance of dolostones, as well as the dolostone/limestone ratio, increase with geologic age. Changes in seawater magnesium concentration were invoked to explain these observations. During the last 30 years, for both scientific and economic reasons, many attempts were made to explain the scarcity of recent dolomites. Carbonate rocks composed mainly of dolomite generally have high porosities, and are important reservoir rocks for oil.
Dolomite is the thermodynamically stable carbonate phase in seawater. Its relative scarcity in recent marine carbonate sediments, therefore, cannot be explained simply by the thermodynamic properties of dolomite; dolomite formation appears to be inhibited by seawater. Most sedimentologists assumed that the formation of dolomite is mainly controlled by the dissolved magnesium/calcium (Mg2+/Ca2+) ratio in seawater; the molar (Mg2+/Ca2+) ratio in seawater is 5.3. It was therefore assumed that dolomite formation requires a still higher ratio. This explanation, however, had to be abandoned when primarily during the last 2 to 3 years, dolomite was observed as an important constituent of many modern marine organic-rich sediments; for example, in th California borderlands, Gulf of California, the Japan Trench, Cariaco basin off the coast of Venezuela, and the Solar Lake in Israel. In the Guaymas basin, for example, dolomite actively forms from pore fluids with Mg2+/Ca2+ ratios of 1 to 2. Similar dolomite formed in the Miocene Monterey Formation, either as penecontemporaneous or early diagenetic nodules or as laminated dolomite. The ^dgr18O values of these dolomites range from -6 to +7 ^pmil (PDB), and their ^dgr13C values range from -30 to +20 ^pmil.
Experiments conducted recently in our laboratory have shown that the important condition of dolomite formation is not high Mg2+/Ca2+ ratios, but low dissolved sulfate (SO4-4) content, which inhibits dolomite formation, seawater contains 28 mMSO42-. Dolomitization of calcite is already inhibited at sulfate concentrations of approximately 5 to 7% of seawater value. Aragonite dolomitization, however, though strongly retarded at these low dissolved SO42- concentration values, is inhibited at sulfate concentrations of approximately 50% of seawater's value. We have also shown that dolomitization of CaCO3 proceeds through protodolomite, a calcium-rich disordered dolomite which would transform t dolomite if equilibrium were established. Dissolved SO42- also retards the rates of protodolomite transformation to dolomite and dedolomitization.
Sulfate ions adsorbed to the surfaces of calcite, dolomite, or aragonite may affect the kinetics of phase transformations in the carbonate system. In a sulfate-depleted environment, sulfate is rapidly desorbed; at 15° to 35°C (59° to 95°F), the process is completed in 1 to 2 days.
Thus, favorable sites for dolomite formation in the marine environment are those where dissolved SO42- concentrations are low. The most effective processes of sulfate removal from or its dilution in marine pore fluids are microbial reduction in organic-rich sediments and mixing of seawater with large amounts of fresh water. This happens, for example, in continental margin sediments with high rates of deposition, as in the Gulf of California, or in deeper water sediments associated with black shales. The presence of much dissolved SO42- in seawater also explains the almost absence of dolomite in apparently highly favorable environments, for example, in open marine carbonates.
The relative paucity of dolomite-rich carbonate rocks (dolostones) formed during the last 100 to 120 million years, as compared with their abundance in older sediments, in particular in the Precambrian and early Paleozoic, could be explained on the basis of changes in the depositional environments and in the carbonate mineralogy of the primary calcareous sediments. These differences do not necessarily reflect changes in seawater magnesium concentrations.
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