Non-Henrian partitioning of nitrogen during slab dehydration

Abstract:

Nitrogen is primarily transported into subduction zones in sediments and the uppermost oceanic crust. Because NH₄⁺ is compatible in slab minerals, while NH₃ and N₂ are less so, the retention N in the slab during dehydration and melting depends on the relative stability of N species as a function of P, T, ƒO₂, and pH. These dependencies make N a potentially information-rich tracer of slab conditions, but the relative stability of N species is also predicted to be a function the total N concentration, given the stoichiometry of converting NH₄⁺ or NH₃ to N₂. Nitrogen partitioning in slab environments may therefore be non-Henrian.

Confirming the prediction of non-Henrian behaviour is crucial because 1) experiments documenting the reactivity of N in slab systems are uniformly run at high concentrations and 2) the natural concentration of N in slabs is likely highly variable, but generally much lower compared to experiments.

We have conducted N partitioning experiments under slab conditions to test for non-Henrian behavior. We completed a series of fixed pressure (1.75 GPa) and temperature (800 °C) piston cylinder experiments (Tulane University) using a double-capsule approach to buffer ƒO₂ (NNO). Within the series, we reacted rhyolitic melt, biotite, and a hydrous fluid with various concentrations of N (3-20 wt. % N). Experiments were analyzed by an electron microprobe (Smithsonian Institution).

Our results reveal that N partitioning between melt and fluid systematically decreases with increasing N concentration. This result is consistent with the non-Henrian behavior predicted by conversion between NH₄⁺ or NH₃ to N₂. Application of our results to systems with natural (low) N concentrations implies an expanded stability of NH₄⁺ and NH₃ relative to N_2. All things being equal, more stable NH₄⁺ should enable more N that is subducted to be ultimately transported into the post-arc mantle. To maintain the strong enrichment of N at Earth’s surface, despite its continuous subduction, we suggest that oxidizing, and potentially high pH conditions, have prevailed during dehydration of the uppermost sections of slabs through time.