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 fin e-grained feldspar aggregates were determined experimentally. The star ting 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 solu tion surrounding the sample, and depth profiling using an ion micropro be (SIMS). From the depth profiles, the product of the grain boundary diffusion coefficient (D') and effective boundary width (delta) was ca lculated using numerical solutions to the appropriate diffusion equati on. The experimental reproducibility of D'delta is a factor of 3. A se parate determination of D' independent of delta yields an effective gr ain boundary width of similar to 3 nm, consistent with high resolution TEM observations of a physical grain boundary width < 5 nm. Oxygen (a s molecular water) grain boundary diffusion rates were determined in t he Ab and Or aggregates at 450 degrees-800 degrees C and 100 MPa (hydr othermal), 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 a ir). 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) th an oxygen, and the Arrhenius relations cross at similar to 625 degrees C. Potassium D'delta values in Or aggregates are about a factor of fi ve greater in hydrothermal experiments at 100 MPa than in experiments at 0.1 MPa in air. Calcium grain boundary diffusion rates in An aggreg ates are 4 to 5 orders of magnitude slower than potassium in Or and ha ve a greater (291 kJ/mol) activation energy. This suggests that differ ences in formal charge and/or size of diffusing species may play an im portant role in their relative grain boundary diffusion rates.