Bimetasomatic zoning in the CaO-MgO-SiO2-H2O-CO2 system: Experiments with the use of natural rock samples

Experiments reproducing the development of bimetasomatic zoning in the CaO-MgO-SiO₂-H₂O-CO₂ system were conducted at elevated P-T parameters with the use of samples of naturally occurring quartzdolomite and calcite-serpentinite rocks. In order to maintain mass transfer exclusively via the diffusion-controlled mechanism, we used the method of the ensured compaction of the cylindrical sample surface with a thin-walled gold tube. In the course of the experiments, a single diopside zone ～2.5 × 10^?5 m thick was obtained at the quartz-dolomite interface at T = 600°C, $P_{H_2 O + CO_2 } $ = 200 MPa, and $X_{CO_2 } $ = 0.5 for 25–40 days and a succession of metasomatic zones at T = 750°C, $P_{H_2 O + CO_2 } $ = 300 MPa, and $X_{CO_2 } $ = 0.4 for 48 days. The metasomatic zones were as follows (listed in order from quartz to dolomite): wollastonite ‖ diopside ‖ tremolite ‖ calcite + forsterite; with the average width of the diopside zone equal to ～1.3 × 10^?5 m and the analogous part of the wollastonite zone equal to ～2.6 × 10^?5 m. Two zones (listed in order from calcite to serpentine) diopside and diopside-forsterite (the average widths of these zones were ～6 × 10^?4 and ～8 × 10^?4 m, respectively) were determined to develop at contact between serpentine and calcite during experiments that lasted 124 days at T = 500°C, $P_{H_2 O + CO_2 } $ = 200 MPa, and $X_{CO_2 } $ = 0.2–0.4. In the former and latter situations, the growth rate of the zoning ranged between 3.1 × 10^?12 and 1.2 × 10^?11 m/s and between 5.6 × 10^?11 and 7.5 × 10^?11 m/s, respectively. The higher growth rate in the latter case can be explained by the higher water mole fraction in the fluid, with this water released during serpentinite decomposition in the experiments. The development of the only diopside zone in the experiments modeling the interaction of quartz and dolomite at T = 600–650°C and $P_{H_2 O + CO_2 } $ = 200 MPa is in conflict with theoretical considerations underlain by the Korzhinskii-Fisher-Joesten model. The interaction of quartz and dolomite in the CaO-MgO-SiO₂-CO₂-H₂O system at the P-T- $X_{CO_2 } $ parameters specified above should be attended by the origin of a number of reaction zones consisting of various proportions of talc, forsterite, tremolite, diopside, and calcite. The saturation of the fluid with respect to these minerals was likely not reached, and this resulted in the degeneration of the respective stability fields in the succession of zones. Conceivably, this was related to the insufficient rates of quartz and dolomite dissolution and the relatively low diffusion rates of the dissolved species in the low-permeable medium. In the experiments with interacting calcite and serpentine, the zoning calcite ‖ diopside ‖ diopside + forsterite ‖ serpentine developed in its complete form, in agreement with the theory. Equilibrium was likely achieved in these experiments due to the higher diffusion coefficients.