Subduction-zone cycling of nitrogen in serpentinized mantle rocks

Nitrogen (N) has shown great potential as a geochemical tracer of volatiles recycling, in part because of large differences in the N isotope composition of the various Earth reservoirs. The subduction flux of N in serpentinized oceanic mantle could be as important as N input flux in oceanic crust and even sediment because, although its N concentrations are lower, its volume is potentially far greater than that of the crust/sediment. However, recycling of oceanic mantle rocks is still poorly constrained for the N cycle, and N isotope data for subduction-related ultramafic rocks are scarce [1]. The primary goal of this study is to characterize the subduction flux of N in subducting altered oceanic mantle by documenting concentrations and isotopic compositions of N in mantle rocks that reflect different stages of the metamorphic subduction zone cycle. The results are crucial to assess the composition of N recycled into the mantle, to determine the extent to which N can be retained in subducted mantle rocks to depths approaching those beneath arcs, and to balance N subduction-zone inputs with outputs in arc volcanic gases. Moreover, information has been gained regarding the redistribution and isotope fractionation of N via ultramafic dehydration and metamorphic fluid-rock interaction. The samples analyzed in this study are ultramafic rocks from shallow oceanic environments to increasing P-T conditions up to depths of ~70 km. Three distinct metamorphic grades, reflecting seafloor fluid uptake, water release due to brucite breakdown and the final antigorite breakdown, were investigated: 1. Pre-subduction serpentinized mantle peridotite from non-subducted ophiolite sequences from the Northern Apennines, Italy (Monte Nero). 2. Eclogite-facies antigorite serpentinites from the Ligurian Alps, Italy (Erro Tobbio). 3. Eclogite-facies chlorite harzburgites derived from dehydration of serpentinites from the Betic Cordillera, Spain (Cerro de Almirez). The pre-subduction peridotites have the lowest N concentrations (1.3-2.1 ppm) and highly variable δ15N values (-4 to +3). High-pressure peridotites have higher N contents (up to 20 ppm) and mostly positive δ15N values. Data for veins in the peridotites are suggestive that N isotopes were not significantly fractionated by dehydration. Hence, N recycled through subduction zones via peridotites should show a range in δ15N similar to that of oceanic serpentinized peridotites. The range in δ15N for serpentinized peridotites, from pristine mantle values towards those typical of modern marine sediments, suggests derivation of N from organic-sedimentary sources, incorporated during bending-related faulting of the subducting slab and/or via metasomatic additions during subduction. N is enriched in serpentinized peridotites compared to pristine mantle rocks, and it is retained down to depths of at least 70 km and possibly deep into the mantle. Hence, serpentinized peridotites appear to represent an important reservoir for deep subduction zone N cycling. [1] Philippot et al. (2007), Min. Pet. 91:11-24