Physics
Scientific paper
Jun 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991gecoa..55.1597f&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 55, Issue 6, pp.1597-1607
Physics
14
Scientific paper
Grain boundary diffusion measurements using fine-grained, natural, monomineralic aggregates offer several distinct advantages over techniques previously employed. Pure quartz aggregates (Arkansas novaculite), with 1.2 and 4.9 m diameter grains, were annealed at 450-800°C and 100 MPa confining pressure in 18 O-enriched water. Profiles of 18 O /( 18 O + 16 O ) with depth from the surface were measured using an ion microprobe, and data were collected from an area 68 m in diameter in order to obtain a well-averaged value for many grain boundaries. Using graphical solutions appropriate to the boundary conditions employed, the product of the average grain boundary diffusion coefficient ( D ') and effective boundary width ( ) is obtained and is independent of the grain size, geometry, and grain boundary tortuosity. Arrhenius parameters for the 1.2 and 4.9 m grain size samples are: D ' 0 = 2.6 and 3.4 × 10 -17 m 3 /sec, and Q = 27 ± 1 and 26 ± 3 kcal/mol, respectively. Measured values of D ' were about three times greater for the 4.9 m aggregate, although this might be due in part to thermal cracking while going to temperature. The activation energy of both samples, ~ 27 kcal/mol ( 113 kJ/mol), is significantly less than that for volume diffusion of oxygen in quartz, but greater than that for ionic diffusion in a static fluid. Measured D ' values are within the range of most previous estimates. For a representative effective grain boundary width of 1 nm, the oxygen grain boundary diffusion coefficients are 4 to 6 orders of magnitude greater than oxygen volume diffusion coefficients in quartz single crystals, and ~6 orders of magnitude less than ionic diffusion coefficients in a static fluid, over the temperature range of the experiments.
Farver John R.
Yund Richard A.
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