Details
| Original language | English |
|---|---|
| Pages (from-to) | 111-118 |
| Number of pages | 8 |
| Journal | Chemical geology |
| Volume | 483 |
| Publication status | Published - 20 Apr 2018 |
| Externally published | Yes |
Abstract
It is generally agreed that decoupling of the Hf and Nd isotope systems in modern aqueous systems is a result of incongruent release of Hf during terrestrial weathering of the continental crust, although the mechanism(s) behind this process are not yet fully understood. We here present Hf–Nd isotope data for the Neoarchean Krivoy Rog Banded Iron Formation (BIFs), Ukraine, and combine observations on modern aqueous environments with those of the early Earth to further evaluate the mechanism(s) behind Hf–Nd isotope decoupling in aqueous systems. The pure Late Archean Krivoy Rog chemical sediment endmember, representing the dissolved pool of ancient seawater, shows decoupled εNd2.60Ga-εHf2.60Ga values of −2.3 and +9.48, respectively, and suggests that decoupled Hf–Nd isotopes had been a global rather than a local phenomenon in Neoarchean seawater. This further reveals that incongruent Hf release via terrestrial weathering and erosion of emerged and evolved continental landmasses were widespread geological processes by Late Archean time. Impure Krivoy Rog BIF samples, composed of a mixture of seawater-derived and detrital Nd and Hf, show systematically more positive εNd2.60Ga values, but still reveal decoupled εNd-εHf values relative to an associated schist that plots slightly below the “terrestrial array”. This suggests that mineral sorting between a zircon-bearing sand-sized fraction and fine-grained sediment particles occurred on/in Late Archean continents, rivers and oceans, and had significant impact on the chemical compositions of the suspended and dissolved element loads of Late Archean seawater. Less radiogenic Hf isotope compositions in the Krivoy Rog seawater relative to detritus-contaminated BIFs further suggest a pathway for high-temperature hydrothermal Hf into anoxic Archean seawater, that diluted the even more radiogenic Hf isotopic composition of continental run-off, created by the mineralogical composition of the continental hinterland and the “zircon effect”. Alternatively, the less pronounced decoupling of εHf-εNd in Late Archean seawater may be related to a shorter residence time of Hf relative to Nd. Furthermore, systematically more positive initial εNd values in detritus-contaminated Archean BIFs relative to respective dissolved seawater loads suggest that weathered and eroded material of (ultra)mafic rock suites had significant impact on the suspended and dissolved fractions in Archean seawater.
Keywords
- Archean, BIF, Hf–Nd isotopes, Seawater, Zircon effect
ASJC Scopus subject areas
- Earth and Planetary Sciences(all)
- Geology
- Earth and Planetary Sciences(all)
- Geochemistry and Petrology
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In: Chemical geology, Vol. 483, 20.04.2018, p. 111-118.
Research output: Contribution to journal › Article › Research › peer review
}
TY - JOUR
T1 - Decoupled Hf and Nd isotopes in suspended particles and in the dissolved load of Late Archean seawater
AU - Viehmann, Sebastian
AU - Bau, Michael
AU - Hoffmann, J. Elis
AU - Münker, Carsten
N1 - Funding information: We strongly acknowledge the generosity and help of Dimitrii Kulik who provided the Krivoy Rog samples to M.B., S.V. and M.B. acknowledge the support from Jacobs University Bremen for a temporary Ph.D. scholarship for S.V. S.V. especially thanks Nathalie Tepe for discussions of elemental behavior during estuarine processes and Katja Schmidt for fruitful discussions about the Hf–Nd isotope systems in aqueous systems. We also want to acknowledge Catherine Chauvel for her editorial handling and two anonymous reviewers for their positive and constructive comments which helped to improve the final version of the manuscript. This study was partially funded by the European Union 's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 746033 and is related to the SPP 1833 “Building a Habitable Earth”.
PY - 2018/4/20
Y1 - 2018/4/20
N2 - It is generally agreed that decoupling of the Hf and Nd isotope systems in modern aqueous systems is a result of incongruent release of Hf during terrestrial weathering of the continental crust, although the mechanism(s) behind this process are not yet fully understood. We here present Hf–Nd isotope data for the Neoarchean Krivoy Rog Banded Iron Formation (BIFs), Ukraine, and combine observations on modern aqueous environments with those of the early Earth to further evaluate the mechanism(s) behind Hf–Nd isotope decoupling in aqueous systems. The pure Late Archean Krivoy Rog chemical sediment endmember, representing the dissolved pool of ancient seawater, shows decoupled εNd2.60Ga-εHf2.60Ga values of −2.3 and +9.48, respectively, and suggests that decoupled Hf–Nd isotopes had been a global rather than a local phenomenon in Neoarchean seawater. This further reveals that incongruent Hf release via terrestrial weathering and erosion of emerged and evolved continental landmasses were widespread geological processes by Late Archean time. Impure Krivoy Rog BIF samples, composed of a mixture of seawater-derived and detrital Nd and Hf, show systematically more positive εNd2.60Ga values, but still reveal decoupled εNd-εHf values relative to an associated schist that plots slightly below the “terrestrial array”. This suggests that mineral sorting between a zircon-bearing sand-sized fraction and fine-grained sediment particles occurred on/in Late Archean continents, rivers and oceans, and had significant impact on the chemical compositions of the suspended and dissolved element loads of Late Archean seawater. Less radiogenic Hf isotope compositions in the Krivoy Rog seawater relative to detritus-contaminated BIFs further suggest a pathway for high-temperature hydrothermal Hf into anoxic Archean seawater, that diluted the even more radiogenic Hf isotopic composition of continental run-off, created by the mineralogical composition of the continental hinterland and the “zircon effect”. Alternatively, the less pronounced decoupling of εHf-εNd in Late Archean seawater may be related to a shorter residence time of Hf relative to Nd. Furthermore, systematically more positive initial εNd values in detritus-contaminated Archean BIFs relative to respective dissolved seawater loads suggest that weathered and eroded material of (ultra)mafic rock suites had significant impact on the suspended and dissolved fractions in Archean seawater.
AB - It is generally agreed that decoupling of the Hf and Nd isotope systems in modern aqueous systems is a result of incongruent release of Hf during terrestrial weathering of the continental crust, although the mechanism(s) behind this process are not yet fully understood. We here present Hf–Nd isotope data for the Neoarchean Krivoy Rog Banded Iron Formation (BIFs), Ukraine, and combine observations on modern aqueous environments with those of the early Earth to further evaluate the mechanism(s) behind Hf–Nd isotope decoupling in aqueous systems. The pure Late Archean Krivoy Rog chemical sediment endmember, representing the dissolved pool of ancient seawater, shows decoupled εNd2.60Ga-εHf2.60Ga values of −2.3 and +9.48, respectively, and suggests that decoupled Hf–Nd isotopes had been a global rather than a local phenomenon in Neoarchean seawater. This further reveals that incongruent Hf release via terrestrial weathering and erosion of emerged and evolved continental landmasses were widespread geological processes by Late Archean time. Impure Krivoy Rog BIF samples, composed of a mixture of seawater-derived and detrital Nd and Hf, show systematically more positive εNd2.60Ga values, but still reveal decoupled εNd-εHf values relative to an associated schist that plots slightly below the “terrestrial array”. This suggests that mineral sorting between a zircon-bearing sand-sized fraction and fine-grained sediment particles occurred on/in Late Archean continents, rivers and oceans, and had significant impact on the chemical compositions of the suspended and dissolved element loads of Late Archean seawater. Less radiogenic Hf isotope compositions in the Krivoy Rog seawater relative to detritus-contaminated BIFs further suggest a pathway for high-temperature hydrothermal Hf into anoxic Archean seawater, that diluted the even more radiogenic Hf isotopic composition of continental run-off, created by the mineralogical composition of the continental hinterland and the “zircon effect”. Alternatively, the less pronounced decoupling of εHf-εNd in Late Archean seawater may be related to a shorter residence time of Hf relative to Nd. Furthermore, systematically more positive initial εNd values in detritus-contaminated Archean BIFs relative to respective dissolved seawater loads suggest that weathered and eroded material of (ultra)mafic rock suites had significant impact on the suspended and dissolved fractions in Archean seawater.
KW - Archean
KW - BIF
KW - Hf–Nd isotopes
KW - Seawater
KW - Zircon effect
UR - http://www.scopus.com/inward/record.url?scp=85042639851&partnerID=8YFLogxK
U2 - 10.1016/j.chemgeo.2018.01.017
DO - 10.1016/j.chemgeo.2018.01.017
M3 - Article
AN - SCOPUS:85042639851
VL - 483
SP - 111
EP - 118
JO - Chemical geology
JF - Chemical geology
SN - 0009-2541
ER -