The influence of mafic and felsic crust on the seawater chemistry ca. 3.0 billion years ago

Author(s): J. Krayer, J. Jodder, A. Hofmann, S. Weyer, M. Willbold, T. Schulz, C. Koeberl, S. Viehmann
Abstract: Banded iron formations (BIFs) are marine chemical sedimentary rocks that serve as prime archives for Precambrian paleo-environmental reconstructions. However, due to the scarcity of well-preserved Archean rocks, the aquatic environments of early Earth remain poorly constrained. In particular, fluxes derived from continents and submarine hydrothermal systems that affected Archean seawater chemistry are crucial for the understanding of the evolution of marine environments. To fill this gap, we present major- and trace element data in combination with Sm-Nd isotopes of individual BIF layers from the ca. 3.0 Ga old Murchison Greenstone Belt (MGB) of South Africa. BIF layers with low immobile element concentrations show seawater-like shale-normalized (subscript SN) rare earth and yttrium (REY_SN) patterns with heavy over light REY_SN enrichment and positive La_SN, Eu_SN, Gd_SN, and Y_SN anomalies, implying an anoxic marine depositional setting with contributions from high-temperature, hydrothermal systems. These BIF samples yield a Sm-Nd age of 2993 ± 97 Ma that overlaps with the proposed depositional age suggesting negligible post-depositional alteration. In contrast, BIF layers with non-seawater-like REY_SN patterns yield a Sm-Nd age of 2504 ± 161 Ma, which can be linked to post-depositional alteration during the ca. 2.7 Ga Limpopo orogeny. The range of initial εNd values from −1.74 to + 0.15 in pristine BIF samples suggests that elements of mixed juvenile and evolved material from emerged continents and/or hydrothermal systems affected Murchison seawater and indicates potential oceanic water mixing in the Murchison region with water masses derived from the northern Pietersburg and the southern Kaapvaal regions.
Organisation(s): Department of Musicology (Historical Musicology, Ethnomusicology and Systematic Musicology), Dean's office of the Faculty of Protestant Theology, Department of Lithospheric Research, Department of Mineralogy and Crystallography, Department of Geology
External organisation(s): Gottfried Wilhelm Leibniz Universität Hannover, University of Oslo, University of Witwatersrand, University of Johannesburg (UJ), Institute of Earth System Sciences, Georg-August-Universität Göttingen
Journal: Precambrian Research
Volume: 418
ISSN: 0301-9268
DOI: https://doi.org/10.1016/j.precamres.2025.107701
Publication date: 03-2025
Peer reviewed: Yes
Austrian Fields of Science 2012: 105105 Geochemistry, 105127 Geochronology
Keywords
ASJC Scopus subject areas: Geology, Geochemistry and Petrology
Portal url: https://ucrisportal.univie.ac.at/en/publications/491470e9-cb0c-4366-bfbc-8e5373882440

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