MSA ROEBLING MEDAL LECTURE: NITROGEN DIFFUSION IN MINERALS, WITH IMPLICATIONS FOR N CYCLING IN THE SOLID EARTH

Despite the geochemical importance of nitrogen, the behavior of this element in the solid Earth is not well understood, particularly in terms of its interactions with major rock-forming minerals. Nitrogen diffusion is a potentially important to the overall picture of solid-Earth cycling, and for this reason we undertook a systematic experimental study of N diffusion in key minerals: quartz, K-feldspar, clinopyroxene and olivine. Nuclear reaction analysis (NRA) was used to depth-profile ¹⁵N diffused into mineral specimens from C-H-O-N vapor or molten NH₄Cl, and/or introduced by ion implantation and mobilized by annealing at high temperature. The overall T range of our data set is ~500-1400°C but is more restricted for individual minerals due to their differing stabilities. Experiments were conducted at near-atmospheric pressure and 1 GPa, usually under f_O2-buffered conditions. Depth profiling of in-diffusion and ion-implantation experiments by NRA yields highly consistent results. The activation energies for N diffusion are similar for olivine, diopside and quartz (~140 kJ/mol), but substantially lower for orthoclase (~100 kJ/mol). Diffusivities at 1 GPa are statistically indistinguishable from low-pressure values. Arrhenius laws for N diffusion differ markedly among the 4 minerals investigated, except for those of olivine and clinopyroxene, which are nearly identical. At ~600°C, D_N ranges from 2.3´10^-20 m²/s for orthoclase to 3.9´10^-23 m²/s for olivine and clinopyroxene.

The broad implications of these results are: 1) N released to the mantle wedge from subducted assemblages equilibrates locally with pyroxene and olivine grains on relatively long times scales of 10⁶ to 10⁸ years; and 2) "biological" N captured in high-grade metamorphic minerals is released by diffusion on the retrograde limb of a metamorphic cycle (i.e., during denudation)—partially or entirely, depending on the mineral and the cooling rate. In general, "biological" N captured in minerals during metamorphism becomes available to biology again following a full metamorphic T-t cycle.

Our in-diffusion experiments provide some insight into N uptake from C-H-O-N vapor or fluid. In the experiments at near-atmospheric pressure, N concentrations range from ~30-50 ppm in quartz, ~60-250 ppm in olivine, and ~100-2600 ppm in orthoclase.