Phanerozoic perturbation of the mantle nitrogen cycle recorded by kimberlites

The surface nitrogen (N) cycle is dominated by biological activity, yet its impact on the deep N cycle is poorly constrained. Although recycled sedimentary N is observed in modern magmatic systems, it remains unknown when it first became identifiable in mantle-derived magmas and to what extent it affects the deep mantle N budget. Kimberlites provide a unique opportunity to study mantle N reservoirs because they may sample the subcontinental lithospheric mantle (SCLM), deeper plume-influenced sources, or both. Here, we present N isotope measurements of olivine-hosted fluid inclusions from 13 kimberlite pipes spanning eight cratons and emplaced from 14 ka to 558 Ma. Kimberlites with near-zero δ 15 N (+0.8 ± 1.1 ‰) are observed throughout the Phanerozoic, whereas isotopically heavier N (+5.3 ± 0.5 ‰) is only observed in kimberlites younger than 180 Ma. Elevated δ 15 N values correlate with K 2 O enrichment, indicating interaction with phlogopite-bearing, sediment-modified lithospheric mantle. K-poor kimberlites with plume-like Ne isotopes have δ 15 N and N 2 / 36 Ar consistent with a primordial deep mantle N reservoir. The emergence of 15 N-enriched signatures in Earth’s surface reservoirs is attributed to increased primary productivity and biomass burial following the Cambrian Explosion, which produced N-rich, isotopically heavy sediments that were subsequently subducted. We hypothesise that 15 N-enriched subduction signatures impacted metasomatic fluids that overprinted the SCLM during the Phanerozoic. These observations provide additional temporal resolution into the link between biosphere evolution and the deep N cycle.