Carbon and oxygen diffusion in calcite: Effects of Mn content and PH2O
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The diffusion rates of carbon and oxygen in two calcite crystals of different Mn contents have been studied between 500 and 800 C in a CO2-H2O atmosphere (PCO2=1-5 bars, PH2O=0.02-24 bars) labeled with 13C and 18O. Isotope concentration gradients within annealed specimens were measured using a secondary ion microprobe by depth profiling parallel and perpendicular to the c axis. Despite the anisotropic structure of calcite, the diffusion of carbon and oxygen are both very nearly isotropic. Least-squares fitting of the carbon data to an Arrhenius relation gives an activation energy of 872 kcal/mole, with D0 terms dependent only slightly upon direction: {Mathematical expression}, {Mathematical expression}. These results are in close agreement with previous determinations. Results for oxygen diffusion, however, give D values much larger than those previously reported for dry conditions; at 650 to 800 C the D values are two orders of magnitude larger. The diffusion of oxygen, unlike carbon, is strongly dependent on water pressure, as well as Mn content, and does not fit an Arrhenius relation over the entire temperature range. On the basis of these observations and considerations of the defect chemistry of calcite, it is proposed that carbon migrates as a Frenkel pair. The diffusion of oxygen, however, appears to be more complicated and may depend upon several simultaneous mechanisms. 1984 Springer-Verlag.
