Quantitative record of the Neoarchean water cycle from a 2.67 Ga magmatic-hydrothermal system, Fennoscandian Shield

Abstract

Given the scarcity of reliable paleoclimate record, the surface temperatures of the first half of Earth’s history remain poorly constrained. Here we show how the climate-sensitive δ18O value of surface precipitation recorded in Archean igneous and hydrothermal formations can help to resolve the state of early Earth climate. The Keivy complex, Kola craton (Fennoscandian Shield), formed via the intrusion of granitic and mafic magmas in the shallow crust at 2.67 Ga, where circulation of meteoric water created a distinct archive of the contemporaneous water cycle. Using whole rock data, mineral separates, and in situ zircon δ18O measurements, we disentangle the reaction mechanisms between the shallow magma and local precipitation. Syn-emplacement hydrothermal alteration produced near-contact lithologies with δ18O values as low as –8‰ recorded in amphiboles, while igneous zircons from granites crystallized from melts with δ18O from +1‰ to +4.5‰. High-precision U-Pb geochronology constrains the granite intrusion at 2673.5 ± 0.3 Ma. Using the Δ17O approach, these rocks reveal that the precipitation had a δ18O value 18‰ lower than the hydrosphere, providing one of the earliest quantitative records of continental precipitation generally compatible with a cold climate at high latitudes.