| Popis: |
Equilibrium stable isotope fractionation factors between carbon dioxide and calcite were determined by direct equilibration of the two phases at 900°C and 12.5 kbar. The results of six three-day runs gave Δ13C(CO2-calcite) and Δ18O(CO2-calcite) fractionations of +2.70± 0.36% and +3.30 ± 0.16%., respectively (2σ). Runs of 22.5, 45, and 90 min; and 3, 6, 12, and 24 h duration were also conducted. Rapid initial recrystallization accompanied by extensive carbon and oxygen isotope exchange ( ⪆40% ) and a very high oxygen/carbon exchange rate ratio (~5) was seen in runs using both natural and synthetic calcite. Subsequent exchange slowed drastically, and the rate of oxygen exchange decreased relative to that of carbon to a constant value of ~3, implicating the involvement of carbonate ion migration in the later, slower stages of exchange. The lower limits of bulk carbon and oxygen diffusion coefficients determined from the partial exchange experiments were 4 × 10−13 and 9× 10−13 (cm2/s), respectively. |
