Oxygen grain boundary diffusion in natural and hot-pressed calcite aggregates

Oxygen grain boundary diffusion rates were experimentally determined in natural (Solnhofen limestone) and hot-pressed calcite aggregates at 300 to 500 degrees C and 100 MPa water pressure. The Solnhofen limestone was pre-annealed for 24 h at 700 degrees C and 100 MPa confining pressure under anhydrous conditions to produce an equilibrium microstructure for the diffusion experiments. The synthetic calcite aggregates were formed by hot isostatic pressing of an intimate mixture of 1-2 mu m calcite grains plus 5 wt% 0.3 mu m alumina grains at 700 degrees C and 200 MPa for 3-10 days. Values for the product of the grain boundary diffusion coefficient (D') and the effective grain boundary diffusion width (delta) were determined from O-18 concentration profiles measured with an ion microprobe. There is no measurable difference between D'delta values obtained for pre-annealed Solnhofen samples and hot-pressed calcite aggregates; the temperature dependence for oxygen grain boundary diffusion at 100 MPa is described by the Arrhenius parameters D-0'delta = 3.8 x 10(-14) m(3)/s and Q = 127 +/- 17 kJ/mol. Comparison of the results of this study with previously published data show that the rate of oxygen grain boundary diffusion in calcite aggregates is four to five orders of magnitude greater than that of volume diffusion in calcite single crystals, and the activation energy is less. Grain boundary diffusion of oxygen in calcite aggregates is five to six orders of magnitude greater than that of calcium, when the latter is extrapolated to the temperature range of this study. In addition, oxygen grain boundary diffusion rates in calcite aggregates are about a factor of 4-10 and similar to 100 times greater than that in feldspar and quartz aggregates, respectively, under similar experimental conditions. (C) 1998 Elsevier Science B.V. All rights reserved.