Microstructures and textures of experimentally altered Bahamian ooids: Implications for reaction mechanisms of dolomitization
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Complete dolomitization of aragonitic ooids from shoals south of Cat Cay, Bahamas was achieved within 450 hours at 197°C in the laboratory. Partially to completely altered ooids were recovered at time steps ranging from 70.5 to 450 hours. The magnesium content and degree of ordering of the Ca−Mg carbonate alteration products increase with increasing run time. After 450 hours of exposure to dolomitizing fluids the dolomite contained 3.7 mole % excess calcium. Ordered stoichiometric dolomite was not formed during these experiments.
The unaltered ooids exhibit a core of peloids or skeletal fragments and a rind of concentrically nested organic laminations that support aragonite rods. Alteration reaction mechanisms include fabric-retentive dissolution and precipitation (replacement), fabric destructive replacement, and passive void-filling precipitation (cementation). The style of alteration was evaluated at scales ranging from the micrometer- to the nanometer-scale and is based on back-scattered electron images and bright-field transmission electron microscope images. Alteration reaction mechanisms vary with the duration of the experiment; fabric-retentive replacement is characteristic of short run-times, whereas fabric-destructive alteration distinguishes longer run-times. Cementation occurs throughout the experiments.
Fabric-retentive alteration requires dissolution and precipitation across reaction fronts that are smaller than the primary fabric. Fabricretentive alteration may take place across broad reaction fronts in sedimentary particles when primary fabrics are defined by organic material. The organic substrate that defines the fabric of the aragonitic ooids used in this study permits fabric preservation across reaction fronts up to tens of microns across. Organic material is not preserved during progressive alteration at elevated temperatures in the laboratory and primary organic fabrics are destroyed.
Ca−Mg carbonate crystals that precipitated within 70.5 hours are microstructurally heterogeneous. Heterogeneities take the form of modulations with wavelengths that range from 2.5 to 7.5 nm. Selected-area electron-diffraction patterns of modulated Ca−Mg carbonate crystals reveal primary (a-reflections) and extra diffraction spots that are situated between the primarya-reflections, but are displaced away from the center. These diffraction spots are similar tod-reflections, but are not symmetrically displayed around thea-reflections. Such features are similar to those identified in natural calcian dolomite samples and therefore shed light on the physiochemical and kinetic factors that control formation of microstructural heterogeneities in natural samples.
Study of experimentally altered particles with organically defined fabrics facilitates interpretation of diagenetic fabrics and compositional and structural domains in ancient organosedimentary particles.
KeywordsDolomite Aragonite Dolomitization Ooids Reaction Front
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