Nitrogen recycling in subducted mantle rocks and implications for the global nitrogen cycle.

The nitrogen concentrations [N] and isotopic compositions of ultramafic mantle rocks that represent various dehydration stages and metamorphic conditions during the subduction cycle were investigated to assess the role of such rocks in deep-Earth N cycling. The samples analyzed record low-grade serpentinization on the seafloor and/or in the forearc wedge (low-grade serpentinites from Monte Nero/Italy and Erro Tobbio/Italy) and two successive stages of metamorphic dehydration at increasing pressures and temperatures (high-pressure (HP) serpentinites from Erro Tobbio/Italy and chlorite harzburgites from Cerro del Almirez/Spain) to allow for the determination of dehydration effects in ultramafic rocks on the N budget. In low-grade serpentinites, \u3b415Nair values ( 123.8 to +3.5 \u2030) and [N] (1.3\u20134.5 \u3bcg/g) are elevated compared to the pristine depleted MORB mantle (\u3b415Nair ~ 125 \u2030, [N] = 0.27 \ub1 0.16 \u3bcg/g), indicating input from sedimentary organic sources, at the outer rise during slab bending and/or in the forearc mantle wedge during hydration by slab-derived fluids. Both HP serpentinites and chlorite harzburgites have \u3b415Nair values and [N] overlapping with low-grade serpentinites, indicating no significant loss of N during metamorphic dehydration and retention of N to depths of 60\u201370 km. The best estimate for the \u3b415Nair of ultramafic rocks recycled into the mantle is +3 \ub1 2 \u2030. The global N subduction input flux in serpentinized oceanic mantle rocks was calculated as 2.3 7 108 mol N2/year, assuming a thickness of serpentinized slab mantle of 500 m. This is at least one order of magnitude smaller than the N fluxes calculated for sediments and altered oceanic crust. Calculated global input fluxes for a range of representative subducting sections of unmetamorphosed and HP-metamorphosed slabs, all incorporating serpentinized slab mantle, range from 1.1 7 1010 to 3.9 7 1010 mol N2/year. The best estimate for the \u3b415Nair of the subducting slab is +4 \ub1 1 \u2030, supporting models that invoke recycling of subducted N in mantle plumes and consistent with general models for the volatile evolution on Earth. Estimates of the efficiency of arc return of subducted N are complicated further by the possibility that mantle wedge hydrated in forearcs, then dragged to beneath volcanic fronts, is capable of conveying significant amounts of N to subarc depths