Nitrogen diffusion in silicate minerals, with implications for nitrogen transport and cycling in the lithosphere

Abstract

Diffusion laws for nitrogen in orthoclase, quartz, olivine and clinopyroxene were determined using a combination of experimental strategies that included: in-diffusion from C-H-O-N vapor; in-diffusion from molten NH4Cl; and ion implantation followed by heating to mobilize N. Most experiments were conducted at pressures near 1 bar, but seven experiments were also run in a piston-cylinder device at Ptotal = 1 GPa. The C-H-O-N-source experiments were “wet” (H2O present); those on ion-implanted samples were dry. All N sources were highly enriched in 15N to avoid spurious 14N signals resulting from surface contamination. Use of 15N also enabled depth profiling by nuclear reaction analysis (NRA) using the 15N(p,αγ)12C reaction, which has a detection limit of ~10 ppm. Nitrogen profiles in three samples were characterized by both NRA and SIMS, with similar results. A total of 17 to 28 diffusion coefficient measurements were made on each of the four minerals (83 in all), yielding results that conform for each mineral to an Arrhenius relation of the form D = D0exp(−Ea/RT). The following values for the pre-exponential constant (D0) and activation energy (Ea) were obtained:Unlabelled TableMinerallog(D0, m2/s)Ea (kJ/mol)T range (°C)Orthoclase−13.9 ± 0.495.4 ± 6.6500–900Quartz−12.4 ± 0.3147.3 ± 5.7550–1150Olivine−14.2 ± 0.3135.2 ± 6.6650–1400Clinopyroxene−14.1 ± 0.3135.6 ± 7.4750–1300The various experimental strategies yield generally indistinguishable results at a given temperature, and no significant effects of total pressure or H2O are discernible in the overall diffusion data set. Experiments that involved in-diffusion from external N sources provide qualitative insight into the compatibility of N in the structures of the four minerals. Nitrogen concentrations attain the highest levels in orthoclase (~100–8000 ppm atomic), with quartz, olivine and clinopyroxene falling in the tens to hundreds of ppm range. The new diffusion laws enable us to model the diffusive release of N acquired at depth in the crust during fluid-absent exhumation and cooling. In general, orthoclase is the least retentive of the minerals investigated, and will lose most of its N during cooling from 600 °C at tectonically typical rates. Olivine and clinopyroxene are the most retentive of N. Quartz is intermediate in retentivity between the mafic minerals and feldspar, but in all cases the extent of N exchange with the surroundings depends critically on the specific t-T path of the mineral of interest. In subduction settings where N-bearing fluid is released to the mantle wedge, diffusion is sufficiently fast to homogenize individual mafic mineral grains with respect to N over geologically plausible time scales, but equilibration on the outcrop or regional scale via volume diffusion is precluded by our data.