PRELIMINARY RESULTS FOR NITROGEN DIFFUSION IN POTASSIUM FELDSPAR

Silicate minerals may serve as hosts for significant amounts of N₂ in the solid Earth, perhaps most importantly as NH₄⁺ replacing K⁺. Little is known, however, about the likely rates and extent of NH₄⁺-K⁺ exchange at crustal pressures and temperatures. To address this shortcoming, we have undertaken a study of N uptake in K-bearing minerals, beginning with K feldspar. Samples of orthoclase from Itrongay, Madagascar were encapsulated in silica glass ampoules with ammonium chloride and placed in a furnace at temperatures ranging from 400° to 800°C. Diffusive uptake profiles of nitrogen were then measured by nuclear reaction analysis (NRA) using the ¹⁵N(p,α γ)¹²C reaction; diffusion constants were calculated by fitting the data to the 1-D, error-function solution to the nonsteady-state diffusion equation for constant surface concentration. In attempts to confirm the assumption that the ammonium ion substitutes for potassium in feldspars, potassium loss was also measured with Rutherford back scattering and electron microprobe analysis. While potassium loss was observed, it was greater than the corresponding nitrogen concentrations in experiments conducted to date. The diffusion data nevertheless exhibit systematic, Arrhenius-type behavior, with diffusivities similar to those for heavy alkali elements at a given temperature. Our data suggest that ammonium-K exchange is effective at deep crustal temperatures in timescales of 1 MYr or greater.