PII S0016-7037(01)00590-7 Nitrogen and argon signatures in 3.8 to 2.8 Ga metasediments: Clues on the chemical state of the Archean ocean and the deep biosphere DANIELE L. PINTI,* ,² KO HASHIZUME, and JUN-ICHI MATSUDA ² Department of Earth and Space Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan (Received June 30, 2000; accepted in revised form February 14, 2001) Abstract—N and Ar elemental and isotopic analyses were conducted on Archean metasediments of Isukasia, West Greenland and Pilbara Craton, Western Australia, in order to investigate the N isotopic evolution during the first half of Earth’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 of metamorphism. The release patterns of N and Ar obtained by high-resolution stepped combustion show the occurrence of at least two trapped components. The first is released at 600°C and it is likely contained in fluid inclusions. N is released together with primordial 36 Ar and shows a  15 N value of -1.3  1.0‰, close to that of modern atmospheric N 2 ( 15 N = 0‰). This component is well preserved in hydrothermal-vent silica deposits of North Pole, Pilbara Craton, and nitrogen may represent ammonium salt dissolved in deep-sea hydrothermal fluids. The second N component, released at temperatures higher than 1000°C, is accompanied by radiogenic 40 Ar*, and shows a  15 N value of -7.4  1.0‰ in a kerogen-rich chert from North Pole, Pilbara Craton. This N is likely biogenic and negative 15 N values may reflect a metabolic isotopic fractionation induced by chemosynthetic bacteria using inorganic NH 4 + contained in hydrothermal fluids. This 15 N-depleted biogenic component may occur in Isukasia Banded Iron Formation ( 15 N -1.7‰), but further data are needed to confirm such a hypothesis. In all other samples, metamorphic-induced Rayleigh distillation has altered the pristine N isotopic signature. Copyright © 2001 Elsevier Science Ltd 1. INTRODUCTION Although the nitrogen cycle in the present biosphere is well documented, little is known about its early evolution, because isotopic records in Archean and Proterozoic rocks are scarce. Only a few data have been published (Hayes et al., 1983; Zhang, 1988; Sano and Pillinger, 1990; Gibson et al., 1985; 1986). Recently, Boyd and Philippot (1998) have analyzed Precambrian ammonium in the Moine metasediments of Scot- land, applying a strict mineralogical control on the N isotopic record, to discriminate indigenous biogenic N from diagenetic N. The Moine metasediments show a N heavy component ( 15 N values from +7.4‰ to +16.2‰;  15 N § {[( 15 N/ 14 N) sample /( 15 N/ 14 N) AIR ]-1} 1000) derived from metamor- phic alteration of an original organic source with  15 N values from +5‰ to +10‰, similar to those of organic N in present- day marine sediments (Peters et al., 1978). This implies that a modern-like nitrogen cycle dominated by nitrate species ex- isted in the Late Proterozoic. Beaumont and Robert (1999) analyzed nitrogen from 74 Archean and Proterozoic kerogens showing a  15 N value increase from -6.2‰ in the Early Archean, to +10‰ in the Late Proterozoic. This evolution has been interpreted as changes in the redox potential of the Earth. Negative  15 N values reflect a metabolic isotopic fractionation in anoxic conditions with microorganisms using the reduced forms of nitrogen, while positive  15 N values illustrate the increase of oxygen after Archean, which promoted the biologic production of nitrate species (NO 3 - ) (Beaumont and Robert, 1999). A knowledge of the isotopic composition of nitrogen within the Archean crust can thus provide evidence on the early biologic activity, and is necessary for understanding the origin of N within the near-surface reservoirs (atmosphere+crust). This latter issue is indeed a matter of debate. Crustal and atmospheric nitrogen is likely outgassed from the mantle (e.g., Marty and Humbert, 1997). However, this hypothesis is not compatible with the isotopic imbalance that exists between the upper mantle ( 15 N =-5  2‰; Marty and Humbert, 1997) and the near-surface reservoirs ( 15 N atmosphere = 0‰,  15 N crust from +1 to +15‰; Javoy, 1998). To explain this imbalance, Javoy (1998) suggests the degassing from mantle of an Ensta- tite Chondrite-like primordial source, with  15 N range from -45‰ to -25‰, successively mixed with a late CI-CM chon- drite-like cold veneer ( 15 N  40‰), recycled to the mantle by subduction. Marty and Humbert (1997) assume the degassing of a 15 N-depleted Enstatite Chondrite-like source followed by hydrodynamic escape of the atmospheric N, which produced a positive isotopic shift, by preferential loss of 14 N. These mod- els have been recently questioned by the discovery of positive  15 N values from +1‰ to +6‰ in the primordial lower mantle (Dauphas and Marty, 1999). This paper presents new N and Ar elemental and isotopic data for well-documented Archean sediments of various meta- morphic grades, to investigate the isotopic variations of nitro- gen during the first half of the Earth’s history. The sediments analyzed are cherts, Banded Iron Formation (BIF hereafter), and one metapelite, sampled from the oldest cratons on the Earth: the 3.8 Ga Isukasia Greenstone Belt, West Greenland and the 3.5 to 2.8 Ga Pilbara Craton, NW Australia (Table 1; *Author to whom correspondence should be addressed (pinti@geol.u- psud.fr.). ² Present address: Laboratoire de Ge ´ochronologie UPS (UMR 8616)- IPGP (UMR 7577), Bat. 504, Universite ´ Paris-Sud, 91405 Orsay Ce- dex, France. Pergamon Geochimica et Cosmochimica Acta, Vol. 65, No. 14, pp. 2301–2315, 2001 Copyright © 2001 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/01 $20.00 + .00 2301