Argon isotopic composition of Archaean atmosphere probes early Earth geodynamics

• Published in: Nature (London) 2013-06, Vol.498 (7452), p.87-90
• Main Authors: Pujol, Magali, Marty, Bernard, Burgess, Ray, Turner, Grenville, Philippot, Pascal
• Format: Article
• Language: English
• Subjects: 704/2151/209; 704/2151/210; 704/2151/213; Analysis; Argon; Atmosphere; Climate variability; Composition; Continental crust; Crust; Earth; Earth Sciences; Geochemistry; Humanities and Social Sciences; Isotopes; letter; Magma; multidisciplinary; Properties; Quartz; Radioactive half-life; Reservoirs; Science; Sciences of the Universe
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature12152

The analysis of the isotopic signature of argon in 3.5-billion-year-old hydrothermal quartz suggests an early development of the continental crust, with implications for climate variability at that time. Archaean atmospheric argon found in ancient quartz Magali Pujol et al . report the analysis of argon in fluid inclusions trapped in 3.5-billion-year-old quartz from the Pilbara Craton in Western Australia. These samples have never experienced extensive metamorphism, and evidence suggests that the inclusions contain a primary mixture of Archaean freshwater and hydrothermal fluid. The authors obtain a palaeo-atmospheric 40 Ar/ 36 Ar ratio that is about half that of the present-day value. The 40 Ar derives from the gradual decay of 40 K, whereas 36 Ar is primordial, having been thoroughly degassed from the mantle early in the Earth's history. These findings are consistent with the formation of an early felsic crust. 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 reservoirs 9 . 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 40 Ar/ 36 Ar value of 143 ± 24, lower than the present-day value of 298.6 (for which 40 Ar has been produced by the radioactive decay of the potassium isotope 40 K, with a half-life of 1.25 billion years; 36 Ar 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.