Release of subducted sedimentary nitrogen throughout Earth's mantle

The dynamic process of subduction represents the principal means to introduce chemical heterogeneities into Earth's interior. In the case of nitrogen (N) - atmosphere's most abundant gas - biological-activity converts N-2 into ammonium ions (NH4+), which are chemically-bound within seafloor sediments and altered oceanic crust that comprise the subducting slab. Although some subducted N re-emerges via arc-related volcanism (Sano et al., 1998; Fischer et al., 2002), the majority likely bypasses sub-arc depths (150-200 km) and supplies the deeper mantle (Li et al., 2007; Mitchell et al., 2010; Johnson and Goldblatt, 2015; Bebout et al., 2016). However, the fate of subducted N remains enigmatic: is it incorporated by the shallow convecting mantle - the source of ridge volcanism, or is the deeper mantle - nominally associated with mantle plumes - its ultimate repository? Here, we present N-He-Ne-Ar isotope data for oceanic basalts from the Central Indian Ridge (CIR)-Reunion plume region to address this issue. All on-axis samples with depleted MORB mantle (DMM) affinities (He-3/He-4 = 8 +/- 1 R-A; Graham, 2002) have low N-isotopes (mean delta N-15 = -2.1 parts per thousand), whereas those with plume-like He-3/He-4 display higher values (mean delta N-15 = 1.3 parts per thousand). We explain these data within the framework of a new mantle reference model to predict a time-integrated net N regassing flux to the mantle of similar to 3.4 x 10(10) mol/yr, with the plume-source mantle representing the preferential destination by a factor of 2-3. The model has implications for the present-day imbalance between N subducted at trenches and N emitted via arc-related volcanism, the N-content of Earth's early atmosphere, as well as relationships between N-2 and the noble gases in mantle reservoirs, including He-3/He-4-delta N-15 relationships in plume-derived lavas.