Dolomitization kinetics in hydrothermal bombs and natural settings
- Michigan State Univ., East Lansing, MI (United States). Dept. of Geological Sciences
During high temperature (150--300 C) dolomitization experiments the rate of dolomitization increases with temperature, reactant surface area, reactant solubility, ionic strength and Mg[sup 2+]/Ca[sup 2+] of the solution. One would predict these results from simple kinetic theory. The discoveries that SO[sub 4][sup 2[minus]] slows the rate, Li[sup +] increases the rate and the induction period is long for dolomitization could not have been predicted. Dolomitization is a three-step reaction: (1) Nucleation--nucleation of very high-Mg calcite (35--40 mole % MgCO[sub 3], VHMC) or nonstoichiometric dolomite is followed by nucleation of more stoichiometric dolomite on CaCO[sub 3]; (2) Induction period--nucleation occurs during the induction period but most of this period is post-nucleation growth of VHMC, nonstoichiometric and stoichiometric dolomite nuclei; (3) Replacement period--CaCO[sub 3] is replaced by VHMC or nonstoichiometric dolomite. The three-phase reaction model is consistent with eight characteristics of natural dolomites: (1) very Ca-rich dolomite is common only in modern dolomites; (2) there is a direct relationship between Mg[sup 2+]/Ca[sup 2+] solution and Mg/Ca ratio in the dolomite; (3) the stoichiometry of dolomites in some ancient rocks is directly related to the percentage of dolomite in the rock; (4) suppression of stoichiometric dolomite nucleation allows the persistence of metastable Ca-Mg-CO[sub 3] phases; (5) dolomite-limestone contacts are often sharp; (6) dolomite selectively replaces fine-grained CaCO[sub 3]; (7) dolomite crystals often have cloudy centers and clear rims; (8) dolomite textures are mainly determined by the crystal size of the reactant.
- OSTI ID:
- 7008082
- Journal Information:
- Journal of Sedimentary Research - Section A; (United States), Journal Name: Journal of Sedimentary Research - Section A; (United States) Vol. 64:3; ISSN XZ288H
- Country of Publication:
- United States
- Language:
- English
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