EXPERIMENTAL AND NATURAL MIMETIC DOLOMITIZATION OF ARAGONITE OOIDS

To better understand fabrics in dolomitized carbonates, aragonitic ooi ds 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 CaCl 2 in Teflon-lined stainless steel bombs at 200-degrees-C and equilibri um vapor pressure. X-ray diffraction and petrographic study were used to characterize compositional and textural changes during dolomitizati on. Reaction progressed from aragonite through unordered Ca-Mg carbona te 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 occurr ed by dissolution of aragonite concurrent with precipitation of rhombo hedral 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 coal escence of dolomite crystals in cortical laminae closely mimics origin al 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 conce ntric fabric originates by aragonite dissolution and early dolomite pr ecipitation within porous cortical laminae. Importantly, these experim ental and natural ''replacement'' fabrics show that fine-scale dolomit e ooid fabrics are not indicative of ''primary'' dolomite precipitatio n. Isotopic compositions of Beck Spring mimetic dolomite ooids are enr iched relative to other dolomitized metastable marine components, incl uding acicular cement (aragonite), carbonate mud (aragonite-rich), and columnar cement (magnesian calcite). These data suggest that (1) init ial reaction of aragonite ooid cortices occurred early in the diagenet ic history in marine-dominated fluid, and (2) selective reaction of ar agonite ooid cortices was promoted by a combination of high solubility , high cortical porosity, and high reactant surface area relative to o ther metastable components. Accordingly, textural and chemical data ga thered from mimetic concentric dolomite ooids may be useful indicators of precursor aragonite and early dolomitization events.