Nitrogen isotopic composition of ammoniated phyllosilicates: case studies from Precambrian metamorphosed sedimentary rocks

Ammonium concentrations in phyllosilicates from Archean and Proterozoic schists and carbonates were evaluated by Fourier Transform Infrared microspectrometry (μFTIR). The δ15Nair values of structural nitrogen in these phases were subsequently measured by laser mass spectrometry. A rapid estimate of the concentration of NH4+ in biotite by μFTIR allows the preselection of the samples to be analyzed for their isotopic composition. Micas from 1.90-2.09 Ga metasediments from Finland and from 1.025-1.500 Ga biotite schists from the Moine Succession in Scotland have high concentrations of structural ammonium (between 176 and 1549 ppm), suggestive of a biological origin for the nitrogen. The heavy nitrogen isotopic signatures (δ15N=+4.9‰ to +19.7‰) of structural NH4+ in these micas are considered consistent with biological denitrification in operation at the time of sedimentation. Evidence for denitrification in the Mesoproterozoic is in agreement with geological and geochemical data for abundant free oxygen in the atmosphere, facilitating the stability of marine NO3−. Sequences of early Archean (ca. 3.8 Ga) garnet-mica schists from the Isua Supracrustal Belt (ISB) in southern West Greenland contain biotite rich in NH4+ (between 233 and 512 ppm) and nitrogen isotopic signatures (δ15N=−1.9‰ to +5.9‰), which possibly point to an early evolution of ammonium assimilation and/or nitrogen fixation. This result appears to be consistent with molecular phylogenetic studies in the bacterial and archaeal domains, which suggest that nitrogen fixation and ammonium assimilation were metabolic features of the last common ancestor (LCA) of all organisms.