GSA Connects 2021 in Portland, Oregon

Paper No. 8-7
Presentation Time: 9:40 AM


ZAKHAROV, David1, ZOZULYA, Dmitry2, RUBATTO, Daniela3, COLON, Dylan4 and MARIN-CARBONNE, Johanna1, (1)Institute of Earth Sciences, University of Lausanne, Lausanne, CH-1015, Switzerland, (2)Geological Institute, Kola Science Centre, Apatity, 184209, Russian Federation, (3)Institute of Geological Sciences, University of Berne, Bern, CH-3012, Switzerland, (4)Department of Earth Sciences, University of Geneva, Geneva, ND 1205

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.