Oceanic serpentinites: A potentially critical reservoir for deep nitrogen recycling

Serpentinized oceanic peridotites might be an important reservoir delivering volatile elements including nitrogen (N) into the mantle via subduction. To determine N sources and estimate the budget of alteration-added secondary N in the oceanic mantle peridotite reservoir, we examined oceanic serpentinites from four Ocean Drilling Program (ODP) sites in the Pacific and Atlantic Oceans. Our results showed that, despite large variation in serpentinization condition (high temperatures up to >350 °C at Holes 895D, 1271B, and 920D; low temperatures <150 °C at Hole 1274A), serpentinites from all sites displayed ubiquitous and similar magnitude of N enrichment (3.2−18.6 ppm) from sediments/seawater sources (δ15N = −3.3‰ to +4.4‰), and these values were significantly elevated relative to the low N concentration (0.04−2.0 ppm) and δ15N value (−5‰ ± 2‰) of the depleted mantle. Based on these data, the serpentinized oceanic mantle is estimated to contribute 0.4 ± 0.2−14.7 ± 6.9 × 109 mol N annually to global subduction zones. Although this flux is smaller than that of subducting sediments (57 × 109 mol·yr−1), comparison between oceanic serpentinites and meta-serpentinites from subduction zones suggests that N can be effectively retained in serpentinites during prograde metamorphism. This implies that the serpentinized slab mantle could be a critical reservoir to deliver N enriched in 15N to the mantle (at least 70 km depth) and potentially to the deepest portions of the mantle sampled by deep-rooted mantle plumes.