Hybrid mantle plumes with recycled and primordial nitrogen: Insights from plume-influenced basaltic glasses from Reykjanes Ridge and Rochambeau Bank

Sources of nitrogen (N) in the plume mantle source remains hotly debated between a primordial origin (i.e., acquired during Earth’s formation) and a recycled origin associated with subduction of surficial material. Although N isotope data for plume-derived magmas are limited, the available data show clear differences in N isotope compositions between plume (δ¹⁵N >-2 ‰) and the depleted mid-ocean-ridge basalt mantle sources (i.e., DMM; δ¹⁵N = -5 ± 2 ‰). Here we present N-isotope and noble gas isotope data from two suites of well-characterized plume-influenced submarine basaltic glasses with high ³He/⁴He: 1) the Rochambeau Bank in the northeastern Lau backarc basin (up to ∼23 R_A, where R_A refers to the atmospheric ³He/⁴He ratio), and 2) the Reykjanes Ridge (up to ∼18 R_A) south of Iceland. These sample suites are associated with different tectonic settings: the Tonga subducting slab interacts with the Samoan plume beneath the Lau backarc basin, whereas the Mid-Atlantic Ridge interacts with the Icelandic plume at the Reykjanes Ridge. The contrasting tectonic settings provide a unique opportunity to decipher both the origin of N in plume mantle sources and the interaction of the plume mantle with other mantle components, including recycled material from subducting slabs and the DMM. Our results show that Rochambeau Bank (δ¹⁵N from +1.3 ‰ to +2.8 ‰) and Reykjanes Ridge samples (δ¹⁵N from -2.3 ‰ to +0.1 ‰) are both characterized by δ¹⁵N values that are enriched relative to the DMM. Rochambeau Bank data are consistent with ternary mixing between the DMM (δ¹⁵N = -5 ± 2 ‰; N₂/³He = 3.7 ± 1.2 × 10⁶; N₂/⁴⁰Ar* = 138±65), a subduction component from the adjacent Tonga slab with recycled N (δ¹⁵N = ∼+3 ‰; N₂/³He = ∼10⁹; N₂/⁴⁰Ar* = ∼5 × 10⁶), and a third component with δ¹⁵N = ∼0 ‰, N₂/³He = ∼2 × 10⁵ and N₂/⁴⁰Ar* = ∼40, which we attribute to the Samoan plume component enriched in primordial N. In contrast, Reykjanes Ridge data, combined with Iceland data, are consistent with ternary mixing among the DMM, plume components with recycled N (δ¹⁵N from 0 ‰ to +6 ‰; N₂/³He = ∼10⁹; N₂/⁴⁰Ar* = ∼5 × 10⁶) and primordial N (δ¹⁵N from -2 ‰ to +2 ‰; N₂/³He = ∼2 × 10⁵; N₂/⁴⁰Ar* = ∼40) endmembers in deep Icelandic mantle plumes. This is consistent with the presence of both recycled and primordial N being intrinsic to the Icelandic mantle plume. By integrating N data from other global plume-influenced samples (i.e., Society, Hawaii, and Central Indian Ridge), we show that the global dataset is consistent with ternary mixing among the DMM, and plume components entrained with both recycled N and primordial N. Nitrogen in deep plume sources is therefore likely hybrid in composition, containing both recycled N from subducting slab and primordial N retained from Earth's early stages of formation. The heavier δ¹⁵N of an apparent primordial N component within plume mantle sources (-2 ‰ to +2 ‰) relative to that of the convecting MORB mantle (∼-5 ‰) potentially requires an addition of ¹⁵N-rich carbonaceous material from the outer solar system to the early accreting Earth.