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 of nitrogen in Earth's mantle has been suggested to be almost entirely surface-derived. Recycling would contribute nitrogen with relatively heavy 15N/14N isotope ratios to Earth's mantle. This could explain why the Earth's mantle 15N/14N isotope ratio is substantially higher than both solar gases and chondritic parent bodies akin to enstatite chondrites. Here, published nitrogen isotope data of mid-ocean ridge and ocean island basalts are compiled and used to evaluate the nitrogen subduction hypothesis. Nitrogen isotope ratios are used in conjunction with published N2/3He and K2O/TiO2 ratios on the same basalts. Assuming that 3He is not recycled, N2/3He ratios are argued to trace nitrogen addition to mantle sources via subduction. Various mantle source enrichments for basalts are tracked with K2O/TiO2 ratios: elevated K2O/TiO2 ratios are assumed to primarily reflect the contributions of recycled components in the basalts mantle sources. The main result of our data compilation is that for most basalts, δ15N and N2/3He remain constant across a vast range of K2O/TiO2 ratios. Mid-ocean ridge basalts have δ15N signatures that are lower than air by ~4‰ and an average N2/3He ratio of 3.7 (±1.2) x106 (95% confidence, n = 30). Published δ15N and N2/3He are invariant across K2O/TiO2 ratios that vary over a factor of ~20. Using estimates of slab K2O/TiO2 and [TiO2], the observed invariant δ15N and N2/3He may be fit with slabs containing ~0.1 ppm N. A mass balance shows that adding ~10% recycled slabs to the convective mantle only raises the N2/3He by <5%. Lavas from Iceland, Galapagos and Hawaii have high 3He/4He and 15N/14N ratios relative to the convective mantle. Only seven samples show nitrogen isotopic signatures that are unaffected by air contamination, although those samples are poorly characterized for N2/3He. The seven basalts show δ15N between −2 and 0‰ that do not vary systematically with K2O/TiO2 ratios that vary over a factor of ~5. The N2/3He ratios of these seven basalts is unknown, but the high 3He/4He mantle may be estimated by combining published N2/36Ar to 3He/36Ar ratios. This yields a N2/3He of 2.3 (±1.2) x 106 (1σ uncertainty). This is indistinguishable from the MORB estimate of 3.7 (±1.2) x 106. Invariant δ15N across variable degrees of mantle enrichments and MORB-like N2/3He for the high 3He/4He mantle are not consistent with nitrogen addition to plume sources with elevated 3He/4He ratios. A δ15N between −2 and 0‰ for plume sources, only marginally higher than MORB, could be a primordial feature of undegassed mantle reservoirs. Nonetheless, nitrogen subduction may have contributed to a specific array of mantle sources, as revealed by the few published data on basalts with low 3He/4He ratios. Lavas from the Society plume with low 3He/4He ratios show an enriched mantle source, and they have elevated δ15N ≥ +0.5‰ and N2/3He > 107. For those, the addition of slabs with concentrations of ~0.1 ppm N to a mantle source can account for the integrated dataset. To summarize, the published data suggest that nitrogen subduction may explain a sub-set of published N isotope data on basalts, but that N recycling has an overall more limited impact on mantle nitrogen than previously thought.