ABSTRACT

To better understand fabrics in dolomitized carbonates, aragonitic ooids were replaced with dolomite in laboratory experiments. Recent ooids from Cat Cay were reacted in a solution of 0.5 M MgCl₂ and 0.4 M CaCl₂ in Teflon-lined stainless steel bombs at 200°C and equilibrium vapor pressure. X-ray diffraction and petrographic study were used to characterize compositional and textural changes during dolomitization.

Reaction progressed from aragonite through unordered Ca-Mg carbonate to ordered dolomite within 180 hr. Reaction products are texturally identical irrespective of composition and order. We believe that near-stoichiometric compositions and ordering were achieved by microscale dissolution and reaction of intermediate phases.

Dolomitization occurred by dissolution of aragonite concurrent with precipitation of rhombohedral Ca-Mg carbonate. Dolomitization occurs first at ooid surfaces, where microsucrosic crystals precipitated and coalesced to form a rind. With time, reaction preferentially proceeded into discrete zones of ooid microporosity, where dolomite successively replaced algal borings, outer ooid cortices, and inner ooid cortices. Precipitation and coalescence of dolomite crystals in cortical laminae closely mimics original ooid structure.

Comparison of experimental fabrics with calcitized, partially dolomitized, and completely dolomitized aragonite ooids of the late Proterozoic Beck Spring Dolomite indicates that mimetic concentric fabric originates by aragonite dissolution and early dolomite precipitation within porous cortical laminae. Importantly, these experimental and natural "replacement" fabrics show that fine-scale dolomite ooid fabrics are not indicative of "primary" dolomite precipitation. Isotopic compositions of Beck Spring mimetic dolomite ooids are enriched relative to other dolomitized metastable marine components, including acicular cement (aragonite), carbonate mud (aragonite-rich), and columnar cement (magnesian calcite). These data suggest that (1) initial reaction of aragonite ooid cortices occurred early in the diagenetic history in marine-dominated fluid, and (2) selective reaction of aragonite ooid cortices was promoted by a combination of high solubility, high cortical porosity, and high reactant surface area relative to other metastable components. Accordingly, textural and chemical data gathered from mimetic concentric dolomite ooids may be useful indicators of precursor aragonite and early dolomitization events.