Anomalous nitrogen isotopes in ultrahigh-pressure metamorphic rocks from the Sulu orogenic belt: Effect of abiotic nitrogen reduction during fluid-rock interaction

•Nitrogen isotope compositions of metamorphic rocks subducted to >120-200 km.•Anomalously low N isotope composition as low as −15.8‰.•A possible record of abiotic N reduction process in fossil continental hydrothermal system.

Modern nitrogen (N) fixation is primarily mediated by biological processes. However, in the early Earth where biological activity was absent or limited, abiotic N reduction in hydrothermal systems is thought to be a key process to transform atmospheric N2 and NOx to ammonium, an essential nutrient to support the emergence of life and also an N form that can be incorporated into rocks. Surprisingly, evidence for abiotic N reduction in the rock record has not been clearly identified. In this study, we reported anomalously low N isotope compositions (δN15 values as low as −15.8‰) of mica samples in ultrahigh-pressure metamorphic rocks from the Donghai area in the Sulu orogenic belt, eastern China. Compared with mica samples with typical crustal δN15 values (3-9‰) in similar metamorphic rocks from the western Dabie orogen, the 15N-depleted mica samples from the Sulu orogen are characterized by significant N enrichment (10 times higher) and extreme 18O depletion (δO18 values as low as −9‰). These features can be best explained by assimilation of N from a source characterized by extremely low δN15 values (less than ∼−16‰). The extremely low δN15 value would be produced by abiotic N reduction during reaction of a meteoric-hydrothermal fluid with crustal rocks before subduction. This observation provides a clue to the occurrence of abiotic N reduction in continental supracrustal rocks and infer that abiotic N reduction process could be a fundamental process driving the geological N cycling in early Earth.