Argon isotopic composition of Archaean atmosphere probes early Earth geodynamics

International audienceUnderstanding the growth rate of the continental crust through time is a fundamental issue in Earth sciences1–8.The isotopic signatures of noble gases in the silicate Earth (mantle, crust) and in the atmosphere afford exceptional insight into the evolution through time of these geochemical reservoirs9. However, no data for the compositions of these reservoirs exists for the distant past, and temporal exchange rates between Earth’s interior and its surface are severely under-constrainedowing to a lack of samples preserving the original signature of the atmosphere at the time of their formation. Here, we report the analysis of argon in Archaean (3.5-billion-year-old) hydrothermal quartz. Noble gases are hosted in primary fluid inclusions containing a mixture of Archaean freshwater and hydrothermal fluid. Our analysis reveals Archaean atmospheric argon with a 40Ar/36Ar value of 143624, lower than the present-day value of 298.6 (for which 40Ar has been produced by the radioactive decay of the potassium isotope 40K, with a half-life of 1.25 billion years;36Ar is primordial in origin). This ratio is consistent with an early development of the felsic crust, which might have had an important role in climate variability during the first half of Earth’s history