  Understanding the atmosphere's composition during the Archean eon is a
fundamental issue to unravel ancient environmental conditions. We show from the
analysis of nitrogen and argon isotopes in fluid inclusions trapped in 3.0-3.5
Ga hydrothermal quartz that the PN2 of the Archean atmosphere was lower than
1.1 bar, possibly as low as 0.5 bar, and had a nitrogen isotopic composition
comparable to the present-day one. These results imply that dinitrogen did not
play a significant role in the thermal budget of the ancient Earth and that the
Archean PCO2 was probably lower than 0.7 bar.
