Oxygen diffusion in sanidine feldspar and a critical appraisal of oxygen isotope-mass-effect measurements in non-cubic materials

摘要

Oxygen exchange experiments have been performed between single crystals of sanidine feldspar and water enriched in O-17 and O-18; temperatures were in the range 550-850 degrees C and total pressure 1kbar. Oxygen isotope diffusion profiles in a direction perpendicular to (001) were determined with an ion microprobe. The diffusion data for both isotopes followed a single Arrhenius relationship described by D = 3.95 x 10(-12) exp(-109.7 +/- 4kJmol(-1)/RT) m(2)s(-1). Combining concentration-distance data for O-17 and O-18 simultaneously for each profile in the form of a double 'inverse error function plot' enabled the ratio D-17/D-18 Of diffusion coefficients to be obtained directly, and the isotope mass effect E(beta)(alpha) to be estimated. At 550 and 650 degrees C E(beta)(alpha) values were in the range 0.83-1.03, suggesting an interstitial mechanism for oxygen diffusion in sanidine feldspar, although the uncertainties (+/-0.10) do not preclude a vacancy mechanism. A methodology and mathematical analysis has been developed for isotope-mass-effect studies using infinite reservoir geometry, but the degree of precision attainable is limited by analytical techniques and procedures.