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

¹ Department of Geological Sciences, University of Tennessee, Knoxville, Tennessee 37996–1410, U.S.A.
² Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831–6110, U.S.A.

Correspondence: ^* E-mail: tlabotka{at}utk.edu

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/f_CO2. 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_C 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 toD_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].