Diffusion of C and O in calcite from 0.1 to 200 MPa

Theodore C. Labotka; David R. Cole; Lee R. Riciputi; Mostafa Fayek

doi:10.2138/am-2004-5-614

Abstract

We measured the diffusivity of C and O in calcite over the pressure range 0.1-200 MPa at 600-800°C in a pure CO₂ atmosphere. The experiments were conducted on single, preannealed crystals of Chihuahuan calcite in an isotopically labeled atmosphere, and the diffusion profiles were measured by secondary ionization mass spectrometry (SIMS). At 800 °C, D_C and D_O are identical at 0.1 MPa at a value of ~10^-13.5 cm²/s. The value of D_C decreases to ~10^-16 cm²/s with an increase in pressure to ~50 MPa and remains at that value to 200 MPa, but D_O remains nearly constant at a value of ~10^-14 cm²/s to 200 MPa. The identical values at low pressure indicate that C and O are migrating together as a carbonate anion. A simple model relates the diffusivity of carbonate anions to the formation of vacancies at the crystal surface, which predicts that D_C ∞ 1/fC_O₂. The prediction matches the observed decrease in D_C with increasing pressure to 50 MPa. The shapes of the diffusion profiles for the low-pressure experiments indicate compositional dependence of D, which also suggests the influence of CO₂ sorption on the diffusivity. The value of D_C at 0.1 MPa can be fitted to the relation D_C = 0.62 exp[(-291 kJ/mol)/RT]. The activation energy is nearly twice the value determined for D_O at 100 MPa, ~166 kJ/mol. The change in slope for log D_C vs. P and the change in E_a between 0.1 and 100 MPa suggest that the migrating C species changes from carbonate anions at low pressure to carbon atoms at P ≥ 50 MPa. The values of D_O at 0.1 MPa can be fitted to D_O = 0.017 exp[(-261 kJ/mol)/RT], approximately the same as for C at 0.1 MPa and similar to the relation for D_O at 100 MPa: D_O = 0.008 exp[(-242 kJ/mol)/RT]

Diffusion of C and O in calcite from 0.1 to 200 MPa

Abstract

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