TY - JOUR

T1 - Chemical evolution of seawater in the Transvaal Ocean between 2426 Ma (Ongeluk Large Igneous Province) and 2413 Ma ago (Kalahari Manganese Field)

AU - Schier, K.

AU - Bau, M.

AU - Smith, A. J.B.

AU - Beukes, N. J.

AU - Coetzee, L. L.

AU - Viehmann, S.

N1 - Funding information: We gratefully acknowledge the help and support of P. Andersson, M. Fischerström and H. Schöberg during the analysis of Sm-Nd isotope compositions at the Laboratory for Isotope Geology (LIG), Swedish Museum of Natural Histroy, Brian Alexander for trace element and Nd isotope analyses of the MA-124 and M2 samples, and Finn Bau for his generous help with microdrilling and sample preparation of the GHEX-97 drill core. This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors, but greatly benefited from discussions within the framework of DFG-SPP 1833. We also gratefully acknowledge support from DST-NRF CIMERA for carrying the costs of a field visit by N. J. Beukes and A. J. B. Smith. We also thank Aquila Resources for allowing the sampling of core material (GHEX-97) and South 32 for the MA-124 and M2 cores. We gratefully acknowledge the help and support of P. Andersson, M. Fischerström and H. Schöberg during the analysis of Sm-Nd isotope compositions at the Laboratory for Isotope Geology (LIG), Swedish Museum of Natural Histroy, Brian Alexander for trace element and Nd isotope analyses of the MA-124 and M2 samples, and Finn Bau for his generous help with microdrilling and sample preparation of the GHEX-97 drill core. This research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors, but greatly benefited from discussions within the framework of DFG-SPP 1833. We also gratefully acknowledge support from DST-NRF CIMERA for carrying the costs of a field visit by N. J. Beukes and A. J. B. Smith. We also thank Aquila Resources for allowing the sampling of core material (GHEX-97) and South 32 for the MA-124 and M2 cores. Reviews of Eva Stüecken and one anonymous reviewer substantially improved this manuscript and were greatly appreciated. Further, Franco Pirajno is thanked for all editorial handling.

PY - 2020/12

Y1 - 2020/12

N2 - 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 (REYSN) patterns of jaspilites intercalated with the volcanic rocks of the Ongeluk large igneous province and directly overlying cherts do not show positive EuSN 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 EuSN anomalies, revealing temporary inflow of water masses strongly affected by high-temperature hydrothermal fluids. After this short episode, the REYSN pattern of Transvaal seawater returned to that of pre-Ongeluk times, showing heavy REYSN enrichment, positive LaSN, GdSN and YSN anomalies, but no CeSN or EuSN anomalies. Higher up in the stratigraphy, the Hotazel Formation shows negative CeSN 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 CeSN 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.

AB - 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 (REYSN) patterns of jaspilites intercalated with the volcanic rocks of the Ongeluk large igneous province and directly overlying cherts do not show positive EuSN 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 EuSN anomalies, revealing temporary inflow of water masses strongly affected by high-temperature hydrothermal fluids. After this short episode, the REYSN pattern of Transvaal seawater returned to that of pre-Ongeluk times, showing heavy REYSN enrichment, positive LaSN, GdSN and YSN anomalies, but no CeSN or EuSN anomalies. Higher up in the stratigraphy, the Hotazel Formation shows negative CeSN 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 CeSN 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.

KW - Hotazel Formation

KW - Kalahari Manganese Field

KW - Nd isotopes

KW - Ongeluk LIP

KW - Rare earth elements and yttrium (REY) in seawater

UR - http://www.scopus.com/inward/record.url?scp=85092933624&partnerID=8YFLogxK

U2 - 10.1016/j.gr.2020.09.001

DO - 10.1016/j.gr.2020.09.001

M3 - Article

AN - SCOPUS:85092933624

VL - 88

SP - 373

EP - 388

JO - Gondwana research

JF - Gondwana research

SN - 1342-937X

ER -