Mineral record of abiotic nitrogen reduction

Abiotic nitrogen reduction (ANR) in hydrothermal systems has been long considered as the most likely mechanism to transform N2, the dominant N species on the pre-life Earth’s surface, into NH4+, an essential compound for abiotic synthesis of large organic molecules facilitating the emergence of life in the early oceans [1]. Mineral-induced ANR process has been demonstrated by laboratory experiments [2-4], but not yet been verified in natural hydrothermal systems. To trace the footprint of ANR, we examined NH4+ contents (expressed as N contents below) and N isotope compositions of coexisting muscovite and biotite minerals in 15 granite samples from a fossil hydrothermal system in Eastern China. The samples were formed in a rifting setting of middle Neoproterozoic, where granite H and O isotopes were modified by continental glacial meltwater at high temperature [5]. Our results indicate that muscovite in the samples always have typical granitic signature of low N contents (30-110 ppm) and high δ15N values (2.5-12.6‰). However, except 2 samples displaying equilibrium NH4+ partitiong and N isotope fractionation between muscovite and biotite, the biotite in all other samples show significant N enrichment (N contents from 110-2400 ppm) coupled with lowered δ15N values (2.4‰ to -6.8‰). This feature can be only explained by a kinetic isotopic effect [6] associated with N reduction by Fe2+ in biotite during fluid-rock interaction. Accordingly, these rocks provide a first geological record for ANR process in natural hydrothermal system.