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

Abstract

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.