An experimental study of the breakdown of dolomite in H2O at 700 °C, 100 MPa

Abstract The occurrence of periclase in contact-metamorphosed dolomite rock at modest temperature and pressure indicates an infiltration of H2O because of the otherwise high temperature required for the reaction dolomite → periclase + calcite + CO2. We conducted experiments on the breakdown of dolomite in the presence of H2O at 700 °C, 100 MPa. Grains of dolomite, 175–250 μm in radius, were heated for up to 32 days (32 d). Rims of multigrain calcite and periclase formed around dolomite cores, with ∼50% of the dolomite replaced in 32 d. The growth of the rim is similar to that for previous experiments conducted with cores of dolomite rock and follows a square root of time (t) relation. The decrease in dolomite-core radius with t suggests that the rate is controlled by diffusion of CO2 and H2O through the reaction rim. The results can be modeled by the topochemical or “shrinking-core” model, in which the unreacted grain core radius shrinks as r=r0−κt with a kinetic parameter κ of 6.76 ± 0.6 × 10−4 μm2/s (0.0213 mm2/y). The model predicts that dolomite grains up to 2 mm in radius would disappear in <200 y. In a rock undergoing metamorphism at the conditions of the experiments, the reaction zone grows to a width ranging from <2 mm to >800 mm, depending on the radius of the grains in the rock. The porosity grows rapidly, and CO2 is released abundantly at the beginning of the reaction near the leading edge of the periclase isograd. The shrinking-core process occurs as long as the intergranular fluid is H2O rich, requiring a modest minimum flux sufficient to displace the evolved CO2. The common replacement of periclase by brucite observed in nature was not seen in our experiments. Rather, the quench phase was the magnesium-carbonate hydrate nesquehonite, suggesting the brucite forms only in very CO2-poor fluid.