GRAIN-BOUNDARY DIFFUSION OF OXYGEN, POTASSIUM AND CALCIUM IN NATURAL AND HOT-PRESSED FELDSPAR AGGREGATES

Grain boundary diffusion rates of oxygen, potassium and calcium in fine-grained feldspar aggregates were determined experimentally. The starting materials were a natural albite rock from the Tanco pegmatite and aggregates hot-pressed from fragments of Amelia albite or Ab, Or and An composition glasses. The technique employed isotopic tracers (O-18, K-41, Ca-42) either evaporated onto the surface or in an aqueous solution surrounding the sample, and depth profiling using an ion microprobe (SIMS). From the depth profiles, the product of the grain boundary diffusion coefficient (D') and effective boundary width (delta) was calculated using numerical solutions to the appropriate diffusion equation. The experimental reproducibility of D'delta is a factor of 3. A separate determination of D' independent of delta yields an effective grain boundary width of similar to 3 nm, consistent with high resolution TEM observations of a physical grain boundary width < 5 nm. Oxygen (as molecular water) grain boundary diffusion rates were determined in the Ab and Or aggregates at 450 degrees-800 degrees C and 100 MPa (hydrothermal), potassium rates in Or aggregates at 450 degrees-700 degrees C both at 0.1 MPa (in air) and at 100 MPa (hydrothermal), and calcium rates in An aggregates at 700 degrees-1100 degrees C and 0.1 MPa(in air). Oxygen grain boundary diffusion rates are similar in all three of the Ab aggregates and in the Or aggregate. Potassium and oxygen depth profiles measured in the same samples yield different D'delta values, confirming a diffusional transport mechanism. Potassium diffusion in the Or aggregate has a greater activation energy (216 vs 78 kJ/mol) than oxygen, and the Arrhenius relations cross at similar to 625 degrees C. Potassium D'delta values in Or aggregates are about a factor of five greater in hydrothermal experiments at 100 MPa than in experiments at 0.1 MPa in air. Calcium grain boundary diffusion rates in An aggregates are 4 to 5 orders of magnitude slower than potassium in Or and have a greater (291 kJ/mol) activation energy. This suggests that differences in formal charge and/or size of diffusing species may play an important role in their relative grain boundary diffusion rates.