科研之友
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摘要
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, delta N-15(air) values (-3.8 to +3.5 parts per thousand) and [N] (1.3-4.5 mu g/g) are elevated compared to the pristine depleted MORB mantle (delta N-15(air) similar to -5 parts per thousand, [N] = 0.27 +/- 0.16 mu g/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 delta N-15(air) 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 delta N-15(air) 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 x 10(8) mol N-2/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 x 10(10) to 3.9 x 10(10) mol N-2/year. The best estimate for the delta N-15(air) 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.
- 出版日期2014-10
