MICROSTRUCTURES AND TEXTURES OF EXPERIMENTALLY DOLOMITIZED BAHAMIAN OOIDS: IMPLICATIONS FOR REACTION MECHANISMS OF DOLOMITIZATION

Complete dolomitization of aragonitic ooids from shoals south of Cat Cay, Bahamas was achieved within 450 hours at 197 degrees C in the laboratory. Dolomite was recovered at time steps ranging from 70.5 to 450 hours. Dolomite stoichiometry and degree of ordering increase with increasing run-time. Stoichiometry was not, however, achieved during these experiments. After 450 hours of exposure to dolomitizing fluids the dolomite contained 3.7 mole % excess calcium.

Ooids, which comprise a core that consists of peloids or skeletal fragments and a rind that consists of concentrically-nested organic laminations that support aragonite rods and needles, are altered by reaction mechanisms that include thin-film, fabric-retentive dissolution and reprecipitation, fabric-destructive dissolution and reprecipitation, 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. Reaction mechanisms of dolomitization vary with the duration of the experiment. Fabric-retentive replacement is characteristic of short run-times, whereas fabric-destructive replacement distinguishes longer run-times.

Fabric-retentive preservation requires dissolution and reprecipitation across reaction fronts that are smaller than the primary fabric. Such fabric-retentive dissolution and reprecipitation often takes place across thin aqueous films. The organic substrate that defines the gross fabric of aragonitic ooids from shoals south of Cat Cay permits fabric preservation across reaction fronts up to tens of microns across.

Calcian dolomite 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 calcian dolomite crystals reveal primary (a-reflections) and extra diffraction spots that are situated between the primary a-reflections, but are displaced away from the center. These diffraction spots are similar to d-reflections, but are not symmetrically displayed around the a-reflections.