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

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, DC and DO are identical at 0.1 MPa at a value of ~10-13.5 cm2/s. The value of DC decreases to ~10-16 cm2/s with an increase in pressure to ~50 MPa and remains at that value to 200 MPa, but DO 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 DC ∞ 1/fCO₂. The prediction matches the observed decrease in DC 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 DC at 0.1 MPa can be fitted to the relation DC = 0.62 exp[(-291 kJ/mol)/RT]. The activation energy is nearly twice the value determined for DO at 100 MPa, ~166 kJ/mol. The change in slope for log DC vs. P and the change in Ea 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 DO at 0.1 MPa can be fitted to DO = 0.017 exp[(-261 kJ/mol)/RT], approximately the same as for C at 0.1 MPa and similar to the relation for DO at 100 MPa: DO = 0.008 exp[(-242 kJ/mol)/RT]