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

International audiencePlate 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 surfacederived. Recycling would contribute nitrogen with relatively heavy 15 N/ 14 N isotope ratios to Earth's mantle. This could explain why the Earth's mantle 15 N/ 14 N 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/ 3 He and K2O/TiO2 ratios on the same basalts. Assuming that 3 He is not recycled, N2/ 3 He 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, d 15 N and N2/ 3 He remain constant across a vast range of K2O/TiO2 ratios. Mid-ocean ridge basalts have d 15 N signatures that are lower than air by ~ 4‰ and an average N2/ 3 He ratio of 3.7 (±1.2) x10 6 (95% confidence, n= 30). Published d 15 N and N2/ 3 He are invariant across K2O/TiO2 ratios that vary over a factor of ~ 20. Using estimates of slab K2O/TiO2 and [TiO2], the observed invariant d 15 N and N2/ 3 He ma