Low-δ18O Neoarchean precipitation recorded in a 2.67 Ga magmatic-hydrothermal system of the Keivy granitic complex, Russia

The modern hydrological cycle includes precipitation of O-18-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-delta O-18 magmatic-hydrothermal system that was emplaced in a shallow crust of the Keivy complex, Kola craton at 2.67 Ga. This study builds a case for syn-emplacement hydrothermal exchange with the local meteoric water that had delta O-18 at least as low as -11 parts per thousand VSMOW. Coupled with the high-latitude position of the Kola craton at the time, this result presents a paleogeographic documentation of continental exposure and a quantitative estimate of the isotope composition of Neoarchean precipitation. We employ detailed delta O-18 mapping using mineral separates and bulk samples, as well as ion microprobe delta O-18 measurements and U-Pb dating of zircon. The spatially extensive O-isotope dataset (n = 63) spans over 120 km in length across different lithological units including the peralkaline granites of the Keivy complex, the hosting gneiss and contact hydrothermal quartzolites. All analyzed samples, including whole rock samples and mineral separates, show delta O-18 below those expected for mantle- and crust-derived magmas. The lowest delta O-18 of ca. -7 parts per thousand are measured in minerals and bulk rock of the altered gneisses near the intrusive contacts. Peralkaline granites returned delta O-18 values ranging between -6 and +5 parts per thousand. Zircon crystals with preserved igneous zoning have low delta O-18 = +2.2 parts per thousand and yield an emplacement age for the magmatic complex of 2672 +/- 7 Ma. Zircon dating from quartzolites at the contact between granites and country gneisses points to fluid-rock interaction at 2.67 and at 1.77-1.79 Ga. We propose that the spatially extensive O-isotope depletion documents a Neoarchean shallow magmatic-hydrothermal system where magmatic assimilation produced the low-delta O-18 granites and the hydrothermal cycling of meteoric water in the ambient crust resulted in formation of low-delta O-18 altered host rocks. Since the lowest delta O-18 value measured in a rock is ca. -7 parts per thousand, the equilibrium fluids had delta O-18 of around -9 parts per thousand under hydrothermal conditions (300-400 degrees C) characteristic for the shallow crust around cooling intrusions. Considering that hydrothermal fluids are shifted 1-2 parts per thousand towards higher values, the contemporaneous precipitation had delta O-18 = -11 parts per thousand or lower. Consequently, the 2.67 Ga Keivy complex is the earliest known intact low-delta O-18 magmatic-hydrothermal system offering a quantitative record of the Neoarchean precipitation that can be used as a tool in paleogeographic reconstructions. (C) 2021 The Author(s). Published by Elsevier B.V.

Automated summary

A newly discovered low-delta O-18 magmatic-hydrothermal system from 2.67 Ga provides a paleogeographic documentation of continental exposure and a quantitative estimate of Neoarchean precipitation. The study showcases the exchange of isotopes between meteoric water and magma at high latitudes.