Résumé
Nitrogen and carbon isotopic compositions, together with mineralogy and trace element geochemistry, were studied in a few kerogen-rich Paleoarchean cherts, a barite and a dolomitic stromatolite belonging to the eastern (Dixon Island Formation) and western (Dresser and Strelley Pool Chert Formations; North Pole Dome and Marble Bar) terranes of Pilbara Craton, Western Australia. The aim of the study was to search for N-15-depleted isotopic signatures, often found in kerogens of this period, and explain the origin of these anomalies. Trace elements suggest silica precipitation by hydrothermal fluids as the main process of chert formation with a contamination from volcanoclastic detritus. This is supported by the occurrence of hydrothermal-derived minerals in the studied samples indicating precipitation temperatures up to 350 degrees C. Only a dolomitic stromatolite from Strelley Pool shows a superchondritic Y/Ho ratio of 72 and a positive Eu/Eu* anomaly of 1.8, characteristic of chemical precipitates from the Archean seawater. The bulk delta C-13 vs. delta N-15 values measured in the cherts show a roughly positive co-variation, except for one sample from the North Pole (PI-85-00). The progressive enrichment in N-15 and C-13 from a pristine source having delta C-13 <= -36 parts per thousand and delta N-15 <= -4 parts per thousand is correlated with a progressive depletion in N-15 content and to variations in Ba/La and Co/As ratios. These trends have been interpreted as a progressive hydrothermal alteration of the cherts by metamorphic fluids. Isotopic exchange at 350 degrees C between NH4(rock)+ and N-2(fluid) may explain the isotopic and elemental composition of N in the studied cherts. However, we need to assume isotopic exchange at 350 degrees C between carbonate C and graphite to explain the large C-13 enrichment recorded. Only sample PI-85-00 shows a large N loss (90%) with a positive delta N-15 value (+11 parts per thousand), while C (up to 120 ppm and delta C-13 -38 parts per thousand) seems to be unaffected. This pattern has been interpreted as the result of devolatilization and alteration (oxidation) of graphite by low-temperature fluids. The N-15-C-13-depleted pristine source has delta N-15 values from -7 parts per thousand to -4 parts per thousand and Ar-40/Ar-36 ratios from 30,000 to 60,000, compatible with an inorganic mantle N source, although the elemental abundance ratios N/C and Ar-40/C are not exactly the same with the mantle source. The component alternatively could be explained by elemental fractionation from metabolic activity of chemolithoautotrophs and methanogens at the proximity to the hydrothermal vents. However, ambiguities between mantle vs organic sources of N subsist and need further experimental work to be fully elucidated. (C) 2009 Elsevier Ltd. All rights reserved.