NEOARCHEAN CONTINENTAL EXPOSURE AND HYDROLOGICAL CYCLE RECORDED IN A 2.67 GA MAGMATIC-HYDROTHERMAL SYSTEM

Modern hydrological cycle includes precipitation of ¹⁸O-depleted meteoric water over continental crust exposed above sea-level. How far back in the geological record can continental exposure be documented remains uncertain, particularly for the Archean. To investigate the extent of Archean continental exposure, we document a newly discovered low δ¹⁸O magmatic-hydrothermal system that was emplaced in a subaerial environment of the Kola craton, Russia at 2.67 Ga. We present evidence for syn-emplacement incorporation of the local meteoric water signature with δ¹⁸O at least as low as -11 ‰ VSMOW. The original meteoric water signature was reconstructed using traditional and triple O isotope approaches, indicating that Archean crust was exposed to high-altitude climate with mean annual temperatures of around 4 °C. We employ detailed δ¹⁸O mapping using mineral separates and bulk samples, as well as ion microprobe δ¹⁸O measurements and U-Pb dating of zircon. The spatially extensive O-isotope dataset spans over 120 km in length across different lithological units including the granitic complex, the hosting gneiss and late-stage contact altered rocks. All analyzed samples show δ¹⁸O below the normal mantle- and crustal derived values with the lowest δ¹⁸O of ca. -7 ‰ measured in altered gneisses and granites near the intrusive contacts. We show that pristine low δ¹⁸O zircon crystals yield an emplacement age for the magmatic complex of 2672±7 Ma. A subset of zircons was dated using CA-ID-TIMS geochronology placing the paleogeograpic record on geological timescale with high precision. Consequently, the studied here 2.67 Ga granitic complex provides the earliest quantitative record of low δ¹⁸O precipitation that reflects mature hypsometry of Earth crust and precipitation patterns compatible with an active hydrological cycle over substantially exposed land.