NEOARCHEAN SURFACE CONDITIONS RECORDED IN THE 2.67 GA LOW-δ18O MAGMATIC-HYDROTHERMAL SYSTEM FROM THE KOLA CRATON

The surface temperatures of the first ½ of Earth’s history remains poorly constrained due to the scarcity of well-preserved environmental proxies. Here we reconstruct the surface conditions at 2.67 Ga using surface water archive derived from hydrothermally altered rocks. The δ¹⁸O of meteoric precipitation recorded in these rocks is sensitive to the surface temperatures of the subaerially exposed crust. Due to the high-temperature reactive circulation around cooling plutons, the meteoric water is recorded in high-temperature refractory silicate minerals with δ¹⁸O values <0 ‰. The 3000 km² Keivy complex of the Kola craton formed via the intrusion of peralkaline granitic and mafic magmas in the low δ¹⁸O hydrothermally altered crust at 2.67 Ga. Using whole rock data, mineral separates, and in situ zircon δ¹⁸O measurements, we disentangle the mechanisms of reaction between the shallow magma and local precipitation, which is rarely available for lithologies of such age. Our measurements document direct meteoric water-rock exchange as well as near-contemporaneous igneous assimilation of altered materials. We investigate in detail how low-δ¹⁸O meteoric water is captured in magmas. Zircon with magmatic ages of 2.67 Ga depict melts with δ¹⁸O between 0 and +3‰ that likely formed via assimilation of near-contact hydrothermally altered rocks. The well-preserved zircon were used to constrain magmatic emplacement at 2673.5 ±0.3 Ma using high-precision U-Pb TIMS geochronology. Hydrothermal alteration produced near-contact altered lithologies with whole rock δ¹⁸O values as low as -7 ‰. The δ¹⁸O–Δ¹⁷O of altered host rocks and assimilated magmas unambiguously fingerprints the surface waters with δ¹⁸O = -18 ± 6‰, providing one of the earliest records of continental precipitation. Today such precipitation is found in high-latitude regions with mean annual temperatures mostly below 0 °C. Given existing paleomagnetic data, at 2674 Ma Kola craton was exposed to such precipitation at high latitudes between 60° and 90°, indicating that the climate was likely accompanied by temperatures below freezing. Our newly derived Neoarchean environmental proxy is in accord ca. 2.7 Ga glacial episodes, permitting the existence of cool climate at high latitudes despite an atmosphere presumably rich in CO₂ and CH₄.