OXYGEN SELF-DIFFUSION IN CALCITE - DEPENDENCE ON TEMPERATURE AND WATER FUGACITY

Oxygen self-diffusion in natural calcite single crystals was studied h ydrothermally at 400-800-degrees-C and 10-350 MPa confining pressure. Diffusion coefficients (D) were determined from O-18 concentration pro files measured with an ion microprobe. At 100 MPa confining pressure, the Arrhenius parameters yield an activation energy (Q) = 173 +/- 6 kJ /mole and pre-exponential factor (D0) = 7 x 10(-9) M2/S over the tempe rature range 400-800-degrees-C, and there is no measurable anisotropy. Constant temperature experiments at 700-degrees-C indicate that there is a strong linear correlation (slope = 0.9) of D with water fugacity over the range 4-240 MPa, consistent with the interpretation that, in the presence of water, the dominant oxygen-bearing transport species in calcite is molecular water. A sample containing 1180 ppm Mn shows a marked increase in observed D values at temperatures below approximat ely 600-degrees-C. However, when pre-annealed at greater-than-or-equal -to 700-degrees-C the D values obtained at 550-degrees-C are within a factor of four of the value obtained for the low (100 ppm) Mn sample. The results of this study confirm that in contrast to carbon, oxygen d iffusion rates in calcite are greatly enhanced when water is present. The difference in the effect of water on D(oxygen) and D(carbon) in ca lcite may provide valuable information for evaluating the role of flui ds in the thermal histories of calcite-bearing rocks.