KINETICS OF THE REACTION DOLOMITE = PERICLASE + CALCITE + CO2: AN EXPERIMENTAL INVESTIGATION OF REACTION AND FLUID TRANSPORT IN ROCK CORES

Contact metamorphic analog experiments were conducted using non-siliceous dolomite rock cores to study the extent of reaction and fluid transport resulting from breakdown of dolomite to produce calcite + periclase.  This reaction results in a reduction of the solid volume of the rock and production of CO₂.  Experiments were carried out using a cold-seal hydrothermal apparatus. The dolomite rock cores were sealed in gold capsules with an aliquot of isotopically enriched water of composition H₂¹⁸O or HD¹⁸O_0.5¹⁶O_0.5. The samples were held at a P=100 MPa and T=650 – 750 °C for durations ranging from a few days to a few months. After experimentation, the cores were sectioned and examined by EMP and SIMS techniques.  Reaction rates were calculated, and for experiments at 700 °C a reaction rate of 2E^-11 mol/cm²s was determined.  Extent of reaction was determined by both point counting and by measurement of reaction produced CO₂ and was found to have a log-linear relationship to time which gives an apparent first-order rate constant of 1.4E^-7s^-1.  SIMS ion imaging shows extensive enrichment of ¹⁸O within reaction products and along dolomite grain boundaries and fractures.  Isotope-ratio analyses along traverses within dolomite grains show an enrichment in ¹⁸O from core to rim.  The lack of Fe, Mn, and Sr mobility within dolomite near reaction grain boundaries indicates that diffusion rather than recrystallization is the primary mechanism for ¹⁸O exchange.  Experiments on reaction rates of dolomite breakdown in mixed volatile fluids with powdered starting materials are difficult to evaluate because of the presence of quench phases.  Our experimental results indicate that rock core experiments are effective in determining surface and transport kinetics.