Nitrogen and argon signatures in 3.8 to 2.8 Ga metasediments: Clues on thechemical state of the Archean ocean and the deep biosphere

N and Ar elemental and isotopic analyses were conducted on Archean metasedi ments of Isukasia, West Greenland and Pilbara Craton, Western Australia, in order to investigate the N isotopic evolution during the first half of Ear th's history. The selected samples are deep-sea sediments and hydrothermal deposits having ages from 3.8 to 2.8 Ga and affected by different degrees o f metamorphism. The release patterns of N and Ar obtained by high-resolutio n stepped combustion show the occurrence of at least two trapped components . The first is released at 600 degreesC and it is likely contained in fluid inclusions. N is released together with primordial Ar-36 and shows a delta N-15 value of -1.3 +/- 1.0 parts per thousand, close to that of modern atm ospheric N-2 (delta N-15 = 0 parts per thousand). This component is well pr eserved in hydrothermal-vent silica deposits of North Pole, Pilbara Craton, and nitrogen may represent ammonium salt dissolved in deep-sea hydrotherma l fluids. The second N component, released at temperatures higher than 1000 degreesC, is accompanied by radiogenic Ar-40*, and shows a delta N-15 valu e of -7.4 +/- 1.0 parts per thousand in a kerogen-rich chert from North Pol e, Pilbara Craton. This N is likely biogenic and negative N-15 values may r eflect a metabolic isotopic fractionation induced by chemosynthetic bacteri a using inorganic NH4+ contained in hydrothermal fluids. This N-15-depleted biogenic component may occur in Isukasia Banded Iron Formation (delta N-15 similar to -1.7 parts per thousand), but further data are needed to confir m such a hypothesis. In all other samples, metamorphic-induced Rayleigh dis tillation has altered the pristine N isotopic signature. Copyright (C) 2001 Elsevier Science Ltd.