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

Halama, R; Bebout, GE; John, T; Scambelluri, M

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


Authors

R Halama

GE Bebout

T John

M Scambelluri



Abstract

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, d15Nair values (-3.8 to +3.5 ‰) and [N] (1.3–4.5 µg/g) are elevated compared to the pristine depleted MORB mantle (d15Nair ~ -5 ‰, [N] = 0.27 ± 0.16 µg/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 d15Nair values and [N] overlapping with low-grade serpentinites, indicating no significant loss of N during metamorphic dehydration and retention of N to depths of 60–70 km. The best estimate for the d15Nair of ultramafic rocks recycled into the mantle is +3 ± 2 ‰. The global N subduction input flux in serpentinized oceanic mantle rocks was calculated as 2.3 × 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 × 1010 to 3.9 × 1010 mol N2/year. The best estimate for the d15Nair of the subducting slab is +4 ± 1 ‰, 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.

Acceptance Date Apr 18, 2012
Publication Date Oct 1, 2014
Publicly Available Date Mar 29, 2024
Journal International Journal of Earth Sciences
Print ISSN 1437-3254
Publisher Springer Verlag
Pages 2081 - 2099
DOI https://doi.org/10.1007/s00531-012-0782-3
Keywords nitrogen, N isotopes, Recycling, ultramafic rocks, subduction
Publisher URL http://link.springer.com/article/10.1007/s00531-012-0782-3

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