Mantle wedge serpentinites as a potential nitrogen reservoir regulating subduction-zone nitrogen recycling and mantle heterogeneity

Abstract

Subduction zone is a unique channel on Earth regulating the long-term exchange of nitrogen (N) between Earth's surface and its interior. Increasing evidence has implied that the serpentinized forearc mantle wedge could be an overlooked sink for slab-derived N that potentially regulates the subduction-zone N cycle but has been poorly studied so far. Here we report the N concentrations and isotope compositions of a suite of lizardite serpentinites, lizardite-antigorite serpentinites, and antigorite serpentinites from the Mianlue tectonic mélange in the Qinling Orogen. These samples cover the serpentine phase variations in serpentinized forearc mantle wedge overlain the subducting slab down to a depth of ∼30 km, and thus provide a unique opportunity to unravel the role of the forearc mantle wedge serpentinization in subduction-zone N cycle. The results show that these serpentinites have significantly higher N concentrations (19.7 to 37.4 ppm) than seafloor serpentinites (3 to 19 ppm), indicating more efficient N uptake during fluid-rock interactions inside the subduction zone. The combined N concentrations and δ¹⁵N values suggest that the N added into these serpentinites was mainly mobilized directly from subducted sediments (with a δ¹⁵N range of -1 ‰ to +10 ‰) with a small portion possibly derived from abiotic N₂ reduction (with δ¹⁵N value down to -13 ‰). Despite the prograde phase change from lizardite to antigorite, these serpentinites display comparable N concentrations and no sign of metamorphic N devolatilization. The Mianlue serpentinites also show comparable N concentrations with the forearc serpentinized peridotites (dominated by lizardite/chrysotile) from the Mariana mud volcanoes and forearc antigorite serpentinites from the Tso Morari ultrahigh pressure unit exhumed from >100 km depth. These observations suggest strong N retention in the mantle wedge serpentinites during prograde metamorphism. If such N enrichment is typical in the serpentinized forearc mantle wedge, it means that up to 1.4^±0.6 × 10⁹ mol·yr^-1 slab N can be incorporated into global forearc mantle wedge. Although this amount only accounts for ∼4 % of the sedimentary N input flux, it may account for a significant portion of the small amount of slab N released during early subduction. This implies that forearc serpentinites may play an unprecedentedly recognized role in subduction-zone N cycle. This role may be even more important in hot subduction zones. If N loss from Catalina Schist (i.e., ∼70 % N loss in the forearc) is employed to represent hot subduction zones, our estimation shows that up to ∼25 % of the lost sedimentary N could be re-fixed in forearc serpentinites. The strong N retention in forearc serpentinites as well as in the minerals after antigorite breakdown (e.g., chlorite, amphibole, clinopyroxene and garnet) facilitates deep recycling of slab N (even in the early Earth) if the serpentinized mantle wedge are dragged down by mantle flow. Alternatively, if the serpentinized mantle wedge is imbricated as a portion of the lithospheric mantle, it will enhance the fertilization (and thus heterogeneity) of the lithospheric mantle.