Grain boundary diffusion of oxygen,potassium and calcium in natural and hot-pressed feldspar aggregates |
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| Authors: | John R Farver Richard A Yund |
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| Institution: | (1) Department of Geological Sciences, Brown University, Providence, RI 02912, USA, US |
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| Abstract: | 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 (18O, 41K, 42Ca) 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 (δ) was calculated using numerical solutions to the appropriate diffusion equation. The experimental reproducibility
of D′δ is a factor of 3. A separate determination of D′ independent of δ yields an effective grain boundary width of ∼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°–800° C and 100 MPa (hydrothermal), potassium
rates in Or aggregates at 450°–700° C both at 0.1 MPa (in air) and at 100 MPa (hydrothermal), and calcium rates in An aggregates
at 700°–1100° 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′δ 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 ∼625° C. Potassium D′δ 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.
Received: 24 December 1993 / Accepted: 16 June 1994 |
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