Nitrogen (N) and its isotopes in serpentinized forearc wedges and implications for N cycling across subduction zones

Abstract We investigated nitrogen (N) sources and incorporation in the shallow forearc of an active subduction zone by studying samples from two serpentinite seamounts (South Chamorro and Conical; Ocean Drilling Program Legs 195 and 125) along the Mariana convergent margin. We report N concentrations and δ15N values for serpentinized peridotites, serpentinite muds, and metabasaltic clasts. All are enriched in N relative to likely ultramafic and mafic protoliths. A modest positive correlation between N and δ15N for the serpentinized peridotites could reflect extents of N infiltration by N-bearing fluids. Mixing calculations identify the source of the fluids as dehydrating metasediments and/or altered oceanic crust from the subducting Pacific slab. Such addition is consistent with that of other fluid mobile elements (B, Li, As, and Cs), but their concentrations show little correlation with N. The general lack of correlation in concentrations of N and major and trace elements complicates the identification of mineral hosts and N residency (e.g., loose adsorption on serpentine minerals). The enrichment of N in these samples indicates that serpentinized mantle wedges could be a globally significant N reservoir. If subducted, this reservoir could provide significant amounts of isotopically heavy N to sub-arc depths and beyond.