Experimental Investigation of the Breakdown of Dolomite and Isotope Transport in Rock Cores at 100 MPa, 650–750 °C

The kinetics of the breakdown reaction dolomite = periclase + calcite + CO2 were investigated using cores of dolomitic marble. Two samples of Reed Dolomite from southwestern Nevada were cut into cylinders approximately 4×6 mm in size.

The cores were sealed in gold capsules with isotopically enriched water (H218O or HD 18O0.5 16 O0.5). The samples were heated in a cold-seal hydrothermal apparatus to 650–750 °C at 100 MPa for durations ranging from 2–59 days. The cores were then sectioned and examined by EMP, XRD, and SIMS techniques. All experiments show some amount of reaction regardless of duration or temperature.

Reaction products occur mainly along grain boundaries, fractures within grains, and along sample edges. Ion images and isotope-ratio analysis indicate that reaction products exchanged with infiltrating fluids. Reaction rates were calculated from measured extents of reaction, which were determined from modal analysis from optical point counting, automated EMP modal counts, and by CO2 yield. At 700 °C, a range of reaction rates from 6.6×10−12 to 2.2×10−10 mols/cm2/s was determined.

The extent of reaction was found to have a log-linear relationship with the square root of time, suggesting a diffusion-controlled rate. Activation energies were 106.5 kJ/mol for coarse-grained samples and 202.0 kJ/mol for fine-grained samples. Initial reaction occurs relatively fast near the surface of dolomite rains, but continued diffusion through the reaction products ultimately controls the rate of dolomite breakdown.