Grain boundary diffusion of oxygen in a macroperthitic feldspar

Abstract

Oxygen transport rates along lamellar boundaries in a macroperthitic feldspar have been measured using an ion microprobe. Perthite samples were exchanged hydrothermally with 18O-enriched water at temperatures of 500° to 700°C and at a pressure of 1 kb. Step-scanning traverses along grain boundaries were made with the ion microprobe on surfaces which were cut to show interior faces of the samples. Volume diffusion profiles were determined from step-scanning traverses as well as from depth-profiles. Volume diffusion coefficients for oxygen as determined by step-scanning are given by D0 = 8.0(+290−7.8) × 10−9cm2-sec−1 and Qv= 20 ± 7 kcal-mol−1. As determined by depth-profiling, volume diffusion coefficients are given by D0 = 1.5(+72−1.3) × 10−8cm2-sec−1 and Qv = 26 ± 3 kcal-mol−1. The products of the lamellar boundary diffusion coefficients times the boundary widths, calculated using the volume diffusion coefficients determined by depth-profiling, are given by D0 = 4.7(+240−4.6) × 10−6cm3-sec−1 and Qb = 43 ± 7 kcal-mol−1. The grain boundary diffusion coefficients measured for oxygen (or possibly water) in the perthite are at least 104 times the volume diffusion coefficients. Due to the tightness of the noncoherent lamellar boundary, these data define a lower bound to grain boundary diffusion coefficients for oxygen and perhaps other species in silicates in hydrothermal systems.