Warm and oxidizing slabs limit ingassing efficiency of nitrogen to the mantle

Abstract

Nitrogen is a major and essential component of Earth's atmosphere, yet relative to other volatile elements, there are relatively few experimental constraints on the pathways by which nitrogen cycles between Earth's interior and exterior. We report mineral-melt and mineral-fluid partitioning experiments to constrain the behavior of nitrogen during slab dehydration and sediment melting processes. Experiments reacted rhyolitic melts with silicate and oxide minerals, in the presence of excess aqueous fluid, over temperatures between 725-925 °C and pressures between 0.2 and 2.3 GPa. Oxygen fugacity ranged between iron metal saturation (∼NNO-5) to that in excess of primitive arc basalts (∼NNO+2). Our experiments demonstrate that hydrous fluid is the preferred phase for nitrogen over minerals (biotite, K-feldspar, and amphibole) and rhyolitic melts across all conditions explored. Relatively large effects of pressure (Δlog(Dmelt−fluidN)/Δ(GPa/K) = 761 ± 68 (1σ), Δlog(Dbiotite−fluidN)/Δ(GPa/K) = 462 ± 169) and moderate effects of oxygen fugacity (Δlog(Dmelt−fluidN)/ΔNNO = -0.20 ± 0.04, Δlog(Dbiotite−fluidN)/ΔNNO = -0.10 ± 0.04) modulate partitioning of nitrogen. We further document negligible partitioning effects related to mineral composition or Cl content of hydrous fluid. Of the minerals investigated, biotite has the largest affinity for N and should control the retention of N in slabs where present. Application of partitioning data to slab dehydration PT paths highlights the potential for highly incompatible behavior (Dbiotite−fluidN < 0.1) from the slab along warmer and oxidized (NNO+1) subduction geotherms, whereas dehydration along reduced and cooler geotherms will extract moderate amounts of nitrogen (Dbiotite−fluidN > 0.1). We find that slab melting is less effective at extracting N from slabs than fluid loss, at least under oxidized conditions (NNO+1). Ultimately, the conditions under which slabs lose fluid strongly affect the distribution of nitrogen between Earth's interior and exterior.