- © 2016 Mineralogical Society of America
Earth’s atmosphere contains 27–30% of the planet’s nitrogen and recent estimates are that about one-half that amount (11–16%) is located in the continental and oceanic crust combined. The percentage of N in the mantle is more difficult to estimate, but it is thought to be near 60%, at very low concentrations. Knowledge of the behavior of N in various fluid-melt-rock settings is key to understanding pathways for its transfer among the major solid Earth reservoirs.
Nitrogen initially bound into various organic materials is transferred into silicate minerals during burial and metamorphism, often as NH4+ substituting for K+ in layer silicates (clays and micas) and feldspars. Low-grade metamorphic rocks appear to retain much of this initial organic N signature, in both concentrations and isotopic compositions, thus in some cases providing a relatively un- or little-modified record of ancient biogeochemical cycling. Devolatilization can release significant fractions of the N initially fixed in crustal rocks through organic diagenesis, during progressive metamorphism at temperatures of ~350–550 °C (depending on pressure). Loss of fractionated N during devolatilization can impart an appreciable isotopic signature on the residual rocks, producing shifts in δ15N values mostly in the range of +2 to +5‰. These rocks then retain large fractions of the remaining N largely as NH4+, despite further heating and ultimately partial melting, with little additional change in δ15N. This retention leads to the storage of relatively large amounts of N, largely as NH4+, in the continental crust. Nitrogen can serve as a tracer of the mobility of organic-sedimentary components into and within the upper mantle.
This contribution focuses on our growing, but still fragmentary, knowledge of the N pathways into shallow to deep continental crustal settings and the upper mantle. We discuss the factors controlling the return of deeply subducted N to shallower reservoirs, including the atmosphere, via metamorphic devolatilization and arc magmatism. We discuss observations from natural rock suites providing tests of calculated mineral-fluid fractionation factors for N. Building on our discussion of N behavior in continental crust, we present new measurements on the N concentrations and isotopic compositions of microporous beryl and cordierite from medium- and high-grade metamorphic rocks and pegmatites, both phases containing molecular N2, and NH4+-bearing micas coexisting with them. We suggest some avenues of investigation that could be particularly fruitful toward obtaining a better understanding of the key N reservoirs and the more important pathways for N cycling in the solid Earth.