Chemical evolution of seawater in the Transvaal Ocean between 2426 Ma (Ongeluk Large Igneous Province) and 2413 Ma ago (Kalahari Manganese Field)

Author(s)

K. Schier, Michael Bau, Albertus J.B. Smith, Nicolas Beukes, L.L. Coetzee, Sebastian Viehmann

Abstract

Drill core and outcrop samples of pure marine chemical sediments (banded iron formation (BIF), manganese formation (MnF), jaspilites, lutites, and cherts) from the transition of the ~2426 Ma old Ongeluk Formation into the 2413 Ma old Hotazel Formation, Transvaal Supergroup, South Africa, reveal remarkable changes of seawater chemistry in the Transvaal Ocean. Similar to pre-Ongeluk chemical sediments, the shale-normalized rare earths and yttrium (REY

_SN) patterns of jaspilites intercalated with the volcanic rocks of the Ongeluk large igneous province and directly overlying cherts do not show positive Eu

_SN anomalies, indicating that high-temperature (>250 °C) hydrothermal fluids did not contribute significantly to the REY budget of ambient waters. However, a 10 cm drill core section in the lower Hotazel Formation is characterized by conspicuous positive Eu

_SN anomalies, revealing temporary inflow of water masses strongly affected by high-temperature hydrothermal fluids. After this short episode, the REY

_SN pattern of Transvaal seawater returned to that of pre-Ongeluk times, showing heavy REY

_SN enrichment, positive La

_SN, Gd

_SN and Y

_SN anomalies, but no Ce

_SN or Eu

_SN anomalies. Higher up in the stratigraphy, the Hotazel Formation shows negative Ce

_SN anomalies in some of the lutites, BIFs and MnFs, reflecting Ce depletion in ambient seawater. All Hotazel lutite, BIF, and MnF samples studied show unradiogenic εNd

_(t) values (−0.5 ± 0.2 to −2.4 ± 0.2), indicating a mostly continental REY source. The REY distribution and Nd isotope data combined suggest that oxidative terrestrial weathering of this continental crustal source supplied most of the dissolved REY to local “Transvaal seawater”. Precipitation of the Hotazel lutites, BIFs and MnFs with negative Ce

_SN anomalies, therefore, suggests that oxic conditions prevailed on the Kaapvaal Craton and in Hotazel seawater already at ~2.413 Ga, i.e. 80 m.y. before the disappearance of mass-independent sulfur isotope fractionation (MIF-S) that defines the Great Oxidation Event at ~2.33 Ga.

Organisation(s)

Department of Geology

External organisation(s)

Jacobs Universität Bremen, University of Johannesburg (UJ)

Journal

Gondwana Research

Volume

88

Pages

373-388

No. of pages

16

ISSN

1342-937X

DOI

https://doi.org/10.1016/j.gr.2020.09.001

Publication date

12-2020

Peer reviewed

Yes

Austrian Fields of Science 2012

105105 Geochemistry

Keywords

ASJC Scopus subject areas

Geology

Portal url

https://ucrisportal.univie.ac.at/en/publications/72c8e5f8-4043-4ec6-875e-a99ce3477d15