Nitrogen isotopic insight into sediment recycling in the southern Lesser Antilles arc: Implication to subduction-zone nitrogen recycling

Abstract

Lattice fixed nitrogen component in arc lavas from Kick'em Jenny (KEJ) was investigated for improving our understanding of two long-standing controversies in the recycling of subducting slab material. One is whether the crustal signatures in the Lesser Antilles arc rocks originated from slab recycling or crustal assimilation. The other is the budget of fixed nitrogen in arc crust, which is still a missing piece in the modeling of geodynamic nitrogen cycle. Our results show that the KEJ basaltic lavas contain variable amounts of labile nitrogen (removable by 400°C heating) and 2.6–5.3 ppm refractory nitrogen with δ15N values of +0.8‰ to +6.6‰. The labile nitrogen was likely introduced on seafloor although the KEJ lavas appear to be strikingly fresh, whereas the refractory nitrogen was likely inherited from their magmatic source. Nitrogen isotopic signature of the refractory nitrogen cannot be explained by magmatic degassing, fractional crystallization and/or crustal assimilation. Instead, the data can be best explained by a mixing between the mantle and two δ15N-distinct endmembers, which are consistent with the two types of subducting sediments (i.e., marl/ooze and shale) recovered from the reference site (DSDP Site 144). This supports sediment recycling into the mantle source of the Lesser Antilles arc. Our modeling also yielded a nitrogen fixation flux of 2.3–4.7 × 103 moles N⋅yr⋅−1km−1 (or 1.8–3.8 × 106 moles N⋅yr−1 for the 800 km arc) in the Lesser Antilles arc crust. This is 2–3 orders of magnitude smaller than the nitrogen input flux into the Lesser Antilles trench solely from subducting sediments (1.3 × 106 moles N⋅yr⋅−1km−1 or 1.0 × 109 moles N⋅yr−1 for the entire 800 km trench). This suggests that arc crust is not a major repository of subducted nitrogen, at least in the Lesser Antilles. Instead, most of the crustal nitrogen subducted to the sub-arc depths is either retained in the subducting slab, stored in the metasomatized lithospheric mantle, and/or released back to the surface by volcanic emission.