Anomalous nitrogen isotopes in ultrahigh-pressure metamorphic rocks from the Sulu orogenic belt: Effect of abiotic nitrogen reduction during fluid-rock interaction
Résumé
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 (δ15N 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 δ15N 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 (δ18O values as low as - 9 ‰). These features can be best explained by assimilation of N from a source characterized by extremely low δ15N values (less than ∼ - 16 ‰). The extremely low δ15N 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.