The origin of nitrogen in Earth's mantle: Constraints from basalts 15N/14N and N2/3He ratios

Plate tectonics is thought to be a major driver of volatile redistribution on Earth. The budget nitrogen in Earth's mantle has been suggested almost entirely surface-derived. Recycling would contribute with relatively heavy 15N/14N isotope ratios mantle. This could explain why the ratio substantially higher than both solar gases and chondritic parent bodies akin enstatite chondrites. Here, published data mid-ocean ridge ocean island basalts are compiled used evaluate subduction hypothesis. Nitrogen conjunction N2/3He K2O/TiO2 same basalts. Assuming that 3He not recycled, argued trace addition sources via subduction. Various source enrichments for tracked ratios: elevated assumed primarily reflect contributions recycled components sources. main result our compilation most basalts, ?15N remain constant across vast range ratios. Mid-ocean have signatures lower air by ~4‰ an average 3.7 (±1.2) x106 (95% confidence, n = 30). Published invariant vary over factor ~20. Using estimates slab [TiO2], observed may fit slabs containing ~0.1 ppm N. A mass balance shows adding ~10% convective only raises <5%. Lavas from Iceland, Galapagos Hawaii high 3He/4He relative Only seven samples show isotopic unaffected contamination, although those poorly characterized N2/3He. between ?2 0‰ do systematically ~5. these unknown, but estimated combining N2/36Ar 3He/36Ar yields 2.3 x 106 (1? uncertainty). indistinguishable MORB estimate 106. Invariant variable degrees MORB-like consistent plume sources, marginally MORB, primordial feature undegassed reservoirs. Nonetheless, contributed specific array as revealed few low Society enriched source, they ? +0.5‰ > 107. For those, concentrations N can account integrated dataset. To summarize, suggest sub-set recycling overall more limited impact previously thought.