Halama, R, Bebout, GE, John, T and Scambelluri, M (2014) Nitrogen recycling in subducted mantle rocks and implications for the global nitrogen cycle. International Journal of Earth Sciences, 103 (7). 2081 - 2099. ISSN 1437-3262

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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, δ15Nair values (−3.8 to +3.5 ‰) and [N] (1.3–4.5 μg/g) are elevated compared to the pristine depleted MORB mantle (δ15Nair ~ −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 δ15Nair 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 δ15Nair 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 δ15Nair 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.

Item Type: Article
Uncontrolled Keywords: nitrogen, N isotopes, Recycling, ultramafic rocks, subduction
Subjects: G Geography. Anthropology. Recreation > GB Physical geography
Divisions: Faculty of Natural Sciences > School of Physical and Geographical Sciences
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Depositing User: Symplectic
Date Deposited: 22 May 2015 11:11
Last Modified: 17 May 2019 12:06
URI: https://eprints.keele.ac.uk/id/eprint/539

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