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

Zakharov, D. O.; Zozulya, D. R.; Rubatto, D. (2022). Low-δ18O Neoarchean precipitation recorded in a 2.67 Ga magmatic-hydrothermal system of the Keivy granitic complex, Russia. Earth and planetary science letters, 578, p. 117322. Elsevier 10.1016/j.epsl.2021.117322

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The 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-O 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 O at least as low as −11‰ 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 O mapping using mineral separates and bulk samples, as well as ion microprobe O 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 O below those expected for mantle- and crust-derived magmas. The lowest O of ca. −7‰ are measured in minerals and bulk rock of the altered gneisses near the intrusive contacts. Peralkaline granites returned O values ranging between −6 and +5‰. Zircon crystals with preserved igneous zoning have low O = +2.2‰ 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-O granites and the hydrothermal cycling of meteoric water in the ambient crust resulted in formation of low-O altered host rocks. Since the lowest O value measured in a rock is ca. −7‰, the equilibrium fluids had O of around −9‰ under hydrothermal conditions (300-400 °C) characteristic for the shallow crust around cooling intrusions. Considering that hydrothermal fluids are shifted 1-2‰ towards higher values, the contemporaneous precipitation had O = −11‰ or lower. Consequently, the 2.67 Ga Keivy complex is the earliest known intact low-O magmatic-hydrothermal system offering a quantitative record of the Neoarchean precipitation that can be used as a tool in paleogeographic reconstructions.

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