Argon isotopic composition of Archaean atmosphere probes early Earth geodynamics

Abstract

Understanding the growth rate of the continental crust through time is a fundamental issue in Earth sciences^{1,2,3,4,5,6,7,8}. 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-constrained owing 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 143 ± 24, 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.