Oxygen diffusion studies. I. A preliminary ion microprobe investigation of oxygen diffusion in some rock-forming minerals

Abstract

The self diffusion of oxygen has been studied in prepared natural crystals of albite, grossular garnet, quartz, and ruffle by isotope exchange with hydrothermal water enriched in 18O, and subsequent analysis by ion microprobe. Measured oxygen diffusion coefficients (D) in quartz (‖c) may be described by D = 1.08 × 10−11 exp(−31.5 kcal/RT) cm2s−1 at 600−750°C and 1 kbar water pressure. For grossular, D = 2.5 × 10−16 cm2s−1 at 1050°C and 8 kbar, and D = 4.8 × 10−17 cm2s−1 at 850 °C and 2 kbar. All ruffle crystals exhibited variable amounts of corrosion, and an approximate diffusion coefficient of D(‖c) = 3.16 × 10−15 cm2s−1 cm2s−1 was obtained at 1050 °C and 1 kbar. Oxygen diffusion coefficients in albite, perpendicular to (001) faces, have been determined as a function of pressure at 600 °C Between 0.5 and 8.0 kbar pressure no systematic variation in the results was observed and most of the data may be described by D = 4.1 (±0.5) × 10−15 cm2s−1. Slow oxygen diffusion rates in quartz and garnet suggest that these minerals should have high ‘closure temperatures’ for oxygen exchange, and may provide reliable oxygen isotope exchange geothermometers.

Estimation of oxygen diffusivity from anion porosity in minerals.

“…8, pyrope has the lowest rates of oxygen diffusion relative to the other garnets un der either anhydrous or hydrothermal conditions. Preliminary data for oxygen diffusivity in grossular at 850 and 1050°C were reported by Freer and Dennis (1982). Their results were am biguous because of the marked dog-leg shape of recast inverse error function profiles, possibly re flecting both volume and grain-boundary diffu sions.…”

“…The Arrhenius parameters for oxygen self-dif fusion in a number of minerals have been experi mentally determined by isotopic exchange under anhydrous "dry" and/or hydrothermal "wet" con ditions. The anhydrous diffusion implies the ex change with a nominally anhydrous gas, either 02 or CO2 (e.g., Anderson, 1969;Muehlenbachs and Kushiro, 1974;Kronenberg et al, 1984;Sharp et al, 1991), whereas the hydrothermal diffusion means the exchange with water enriched in 180 at high pressures (e.g., Giletti et al, 1978;Freer and Dennis, 1982;Farver and Giletti, 1985;Fortier and Giletti, 1991). More than two sets of experimental data have been reported for some of the minerals (e.g., quartz, forsterite) under the same conditions.…”

“…More than two sets of experimental data have been reported for some of the minerals (e.g., quartz, forsterite) under the same conditions. Freer and Dennis (1982) recognized that oxygen diffusion under anhydrous conditions was associ ated with slow diffusivities and high activation energies while oxygen diffusion under hydrother mal conditions was associated with faster diffu sion rates and small activation energies. Table 2 lists the representative data plotted in Fig.…”

Diffusion in garnet: a review

“…It is worth noting that their plots of the inverse error function against penetration distance show two distinct linear regions. Freer and Dennis (1982) argued that the near surface region was probably the more reliable one for extracting lattice diffusion rates. However, the small activation energy of *101 kJ/mol obtained from fitting only two data points begged confirmation.…”

“…The experiment found no dependence of diffusion rates on the presence of grain boundaries. Freer and Dennis (1982) measured oxygen self-diffusion rates in a natural grossular garnet at 1123 K and 200 MPa and at 1323 K and 800 MPa under hydrothermal conditions using 18 O-enriched water. It is worth noting that their plots of the inverse error function against penetration distance show two distinct linear regions.…”