The influence of mafic and felsic crust on the seawater chemistry ca. 3.0 billion years ago: Evidence from Nd isotopes in banded iron formations from the Murchison Greenstone Belt

authored by

Johanna Katharina Krayer, Jaganmoy Jodder, Axel Hofmann, Stefan Weyer, Matthias Willbold, Toni Schulz, Christian Koeberl, Sebastian 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)

Mineralogy Section
Geochemistry
Institute of Mineralogy
Institute of Earth System Sciences
Leibniz Research Centre FZ:GEO

External Organisation(s)

University of Oslo
University of the Witwatersrand
University of Johannesburg
University of Göttingen
University of Vienna

Type

Article

Journal

Precambrian research

Volume

418

ISSN

0301-9268

Publication date

27.01.2025

Publication status

E-pub ahead of print

Peer reviewed

Yes

ASJC Scopus subject areas

Geology, Geochemistry and Petrology

Sustainable Development Goals

SDG 14 - Life Below Water

Electronic version(s)

https://doi.org/10.1016/j.precamres.2025.107701 (Access: Open)