Details
Original language | English |
---|---|
Pages (from-to) | 322-334 |
Number of pages | 13 |
Journal | Journal of African earth sciences |
Volume | 111 |
Publication status | Published - 1 Nov 2015 |
Externally published | Yes |
Abstract
Pure marine chemical sediments, such as (Banded) Iron Formations, (B)IFs, are archives of geochemical proxies for the composition of Precambrian seawater and may provide information about the ancient hydrosphere-atmosphere system. We here present rare earths and yttrium (REY) and high precision Sm-Nd isotope data of ~2.90 Ga old Superior-type BIFs from the Witwatersrand Supergroup, South Africa, and compare those with data for near-contemporaneous BIFs from the correlative Pongola Supergroup (Superior-type BIF) and from the Pietersburg Greenstone Belt (Algoma-type IF), respectively. All Witwatersrand samples studied display the typical general REY distribution of Archean seawater, but their REY anomalies are less pronounced and their immobile element concentrations are higher than those of other pure (B)IFs. These observations indicate the presence of significant amounts of detrital aluminosilicates in the Witwatersrand BIFs and question the reliability of the Contorted Bed and Water Tower BIFs (Parktown Formation, West Rand Group) as archives of Mesoarchean seawater. Significant post-depositional alteration of the REY budget and the Sm-Nd isotope system is not observed. The Nd isotopic compositions of the purest BIF samples, i.e. the most reliable archives for Witwatersrand seawater, show initial εNd values between -3.95 and -2.25. This range is more negative than what is observed in ambient shales, indicating a decoupling of suspended and dissolved loads in the "near-shore" Witwatersrand Basin seawater. However, εNd range overlaps with that of the correlative Pongola BIF (Alexander et al., 2008). The deeper-water Algoma-type Pietersburg BIF shows more positive (i.e. more mantle-like) εNd2.9Ga values, supporting the hypothesis that a significant amount of its REY inventory was derived from black smoker-style, high-temperature hydrothermal fluids that had altered seafloor basalts. In marked contrast, the dissolved REY budgets (including the Nd isotopic compositions) of the Witwatersrand and Pongola seawater, however, were dominated by similar terrigenous REY sources from the Kaapvaal Craton.
Keywords
- Archean, BIF, Nd isotopes, REE, Seawater, Witwatersrand
ASJC Scopus subject areas
- Earth and Planetary Sciences(all)
- Geology
- Earth and Planetary Sciences(all)
- Earth-Surface Processes
Sustainable Development Goals
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In: Journal of African earth sciences, Vol. 111, 01.11.2015, p. 322-334.
Research output: Contribution to journal › Article › Research › peer review
}
TY - JOUR
T1 - The reliability of ~2.9 Ga old Witwatersrand banded iron formations (South Africa) as archives for Mesoarchean seawater
T2 - Evidence from REE and Nd isotope systematics
AU - Viehmann, Sebastian
AU - Bau, Michael
AU - Smith, Albertus J.B.
AU - Beukes, Nicolas J.
AU - Dantas, Elton L.
AU - Bühn, Bernhard
N1 - Funding information: This research was financially supported by a much-appreciated Ph.D. scholarship from Jacobs University Bremen and by a travel grant from Deutscher Akademischer Austausch Dienst (DAAD) D/11/44150 to S.V. We acknowledge the kind assistance of Karin Voll and the Geochronology Lab team at the University of Brasilia. We thank Patrick Eriksson for the editorial handling of this manuscript and Claudio Gaucher and Balz Kamber for their constructive reviews which improved the manuscript.
PY - 2015/11/1
Y1 - 2015/11/1
N2 - Pure marine chemical sediments, such as (Banded) Iron Formations, (B)IFs, are archives of geochemical proxies for the composition of Precambrian seawater and may provide information about the ancient hydrosphere-atmosphere system. We here present rare earths and yttrium (REY) and high precision Sm-Nd isotope data of ~2.90 Ga old Superior-type BIFs from the Witwatersrand Supergroup, South Africa, and compare those with data for near-contemporaneous BIFs from the correlative Pongola Supergroup (Superior-type BIF) and from the Pietersburg Greenstone Belt (Algoma-type IF), respectively. All Witwatersrand samples studied display the typical general REY distribution of Archean seawater, but their REY anomalies are less pronounced and their immobile element concentrations are higher than those of other pure (B)IFs. These observations indicate the presence of significant amounts of detrital aluminosilicates in the Witwatersrand BIFs and question the reliability of the Contorted Bed and Water Tower BIFs (Parktown Formation, West Rand Group) as archives of Mesoarchean seawater. Significant post-depositional alteration of the REY budget and the Sm-Nd isotope system is not observed. The Nd isotopic compositions of the purest BIF samples, i.e. the most reliable archives for Witwatersrand seawater, show initial εNd values between -3.95 and -2.25. This range is more negative than what is observed in ambient shales, indicating a decoupling of suspended and dissolved loads in the "near-shore" Witwatersrand Basin seawater. However, εNd range overlaps with that of the correlative Pongola BIF (Alexander et al., 2008). The deeper-water Algoma-type Pietersburg BIF shows more positive (i.e. more mantle-like) εNd2.9Ga values, supporting the hypothesis that a significant amount of its REY inventory was derived from black smoker-style, high-temperature hydrothermal fluids that had altered seafloor basalts. In marked contrast, the dissolved REY budgets (including the Nd isotopic compositions) of the Witwatersrand and Pongola seawater, however, were dominated by similar terrigenous REY sources from the Kaapvaal Craton.
AB - Pure marine chemical sediments, such as (Banded) Iron Formations, (B)IFs, are archives of geochemical proxies for the composition of Precambrian seawater and may provide information about the ancient hydrosphere-atmosphere system. We here present rare earths and yttrium (REY) and high precision Sm-Nd isotope data of ~2.90 Ga old Superior-type BIFs from the Witwatersrand Supergroup, South Africa, and compare those with data for near-contemporaneous BIFs from the correlative Pongola Supergroup (Superior-type BIF) and from the Pietersburg Greenstone Belt (Algoma-type IF), respectively. All Witwatersrand samples studied display the typical general REY distribution of Archean seawater, but their REY anomalies are less pronounced and their immobile element concentrations are higher than those of other pure (B)IFs. These observations indicate the presence of significant amounts of detrital aluminosilicates in the Witwatersrand BIFs and question the reliability of the Contorted Bed and Water Tower BIFs (Parktown Formation, West Rand Group) as archives of Mesoarchean seawater. Significant post-depositional alteration of the REY budget and the Sm-Nd isotope system is not observed. The Nd isotopic compositions of the purest BIF samples, i.e. the most reliable archives for Witwatersrand seawater, show initial εNd values between -3.95 and -2.25. This range is more negative than what is observed in ambient shales, indicating a decoupling of suspended and dissolved loads in the "near-shore" Witwatersrand Basin seawater. However, εNd range overlaps with that of the correlative Pongola BIF (Alexander et al., 2008). The deeper-water Algoma-type Pietersburg BIF shows more positive (i.e. more mantle-like) εNd2.9Ga values, supporting the hypothesis that a significant amount of its REY inventory was derived from black smoker-style, high-temperature hydrothermal fluids that had altered seafloor basalts. In marked contrast, the dissolved REY budgets (including the Nd isotopic compositions) of the Witwatersrand and Pongola seawater, however, were dominated by similar terrigenous REY sources from the Kaapvaal Craton.
KW - Archean
KW - BIF
KW - Nd isotopes
KW - REE
KW - Seawater
KW - Witwatersrand
UR - http://www.scopus.com/inward/record.url?scp=84940469847&partnerID=8YFLogxK
U2 - 10.1016/j.jafrearsci.2015.08.013
DO - 10.1016/j.jafrearsci.2015.08.013
M3 - Article
AN - SCOPUS:84940469847
VL - 111
SP - 322
EP - 334
JO - Journal of African earth sciences
JF - Journal of African earth sciences
SN - 1464-343X
ER -