| Summary: | Fluids trapped in inclusions in well-characterized Archaean hydrothermal quartz crystals were analyzed by the extended argon–argon method, which permits the simultaneous measurement of chlorine and potassium concentrations. Argon and nitrogen isotopic compositions of the trapped fluids were also determined by static mass spectrometry. Fluids were extracted by stepwise crushing of quartz samples from North Pole (NW Australia) and Barberton (South Africa) 3.5–3.0-Ga-old greenstone belts. The data indicate that fluids are a mixture of a low salinity end-member, regarded as the Archaean oceanic water, and several hydrothermal end-members rich in Cl, K, N, and radiogenic parentless ^(40)Ar. The low Cl–K end-member suggests that the salinity of the Archaean oceans was comparable to the modern one, and that the potassium content of the Archaean oceans was lower than at present by about 40%. A constant salinity of the oceans through time has important implications for the stabilization of the continental crust and for the habitability of the ancient Earth. © 2018 Académie des sciences. Published by Elsevier Masson SAS.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Received 27 November 2017, Accepted 12 December 2017, Available online 2 April 2018. This study was funded by the European Research Council (FP/7 2007–2013, grant agreement 267255). Ray Burgess is thanked for mentorship and assistance during Ar–Ar experiments. Samples were made available through collaborations with Pascal Philippot, Axel Hofmann, and Nick Arndt. We are grateful to Mark Kendricks and Romain Tartèse for detailed and constructive comments, and to Marc Chaussidon for edition and suggestions. This is CRPG contribution # 2558. Published - 1-s2.0-S1631071318300221-main.pdf
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