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

Halama, Ralf, Bebout, G. E., John, Timm 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. DOI 10.1007/s00531-012-0782-3.

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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, 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.

Document Type: Article
Keywords: Petrology; Geochemistry; Nitrogen – N isotopes – Recycling – Ultramafic rocks – Subduction
Research affiliation: OceanRep > SFB 574 > C1
OceanRep > SFB 574
Kiel University
Refereed: Yes
DOI etc.: 10.1007/s00531-012-0782-3
ISSN: 1437-3254
Contribution Number:
ProjectNumber
SFB 574222
Date Deposited: 01 Aug 2011 09:18
Last Modified: 08 Oct 2014 12:42
URI: http://eprints.uni-kiel.de/id/eprint/11976

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