219 Mineralogical Society of America (MSA) Awards Lectures, Presidential Address, and Annual Business Meeting

Tuesday, 6 November 2018
3:00 PM - 5:30 PM
Sagamore Ballroom 5

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

Paper number: 219-2
Program: Technical Sessions
Time: 3:40 PM – 4:20 PM

Authors

Abstract

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 fO2-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, DN 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.

Geological Society of America Abstracts with Programs. Vol. 50, No. 6

doi: 10.1130/abs/2018AM-320541

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