DIFFUSION OF O IN DOLOMITE AND MAGNESITE AT 0.1 MPA

The self diffusion of O atoms in the carbonate minerals dolomite and magnesite was measured at a pressure of approximately 0.1 MPa and in the temperature range of 500–625 °C. Single crystals of the minerals were pre-annealed in a CO₂ atmosphere for 24 h. The crystals were encapsulated in platinum, the capsules were evacuated, and CO₂ was transferred to the capsule before welding. The CO₂ had the isotopic composition of ∼98% ¹³C and 90% ¹⁸O. The samples were heated in a muffle furnace for periods ranging from 61 to 74 d. After experimentation, the ¹⁸O/(¹⁸O + ¹⁶O) ratio was measured on a cleavage surface by secondary ionization mass spectrometry depth profiling. The exchanged profile in dolomite is 75 to 200 nm thick, indicating diffusivities of 1.05×10⁻¹⁷ cm²/s at 600 °C to 9.33×10⁻¹⁹ cm²/s at 500 °C . Profiles in magnesite are much shallower, 25–40 nm at 500 and 600 °C, respectively. The data are noisy and give values of D of 2.8–3.1×10⁻¹⁹ for both temperatures. The diffusivities of O in dolomite are similar to those in calcite, measured previously, but are about an order of magnitude greater than that in magnesite.