A refined solution to Earth's hidden niobium: implications for evolution of continental crust and mode of core formation
New high-precision niobium (Nb) and tantalum (Ta) concentration data are presented for early Archaean metabasalts, metabasaltic komatiites and their erosion products (mafic metapelites) from SW Greenland and the Acasta gneiss complex, Canada. Individual datasets consistently show sub-chondritic Nb/T...
Published in: | Precambrian Research |
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Main Authors: | , , , |
Other Authors: | , |
Format: | Article in Journal/Newspaper |
Language: | English |
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Elsevier BV
2003
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Subjects: | |
Online Access: | https://espace.library.uq.edu.au/view/UQ:66654 |
id |
ftunivqespace:oai:espace.library.uq.edu.au:UQ:66654 |
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record_format |
openpolar |
institution |
Open Polar |
collection |
The University of Queensland: UQ eSpace |
op_collection_id |
ftunivqespace |
language |
English |
topic |
Geosciences Multidisciplinary Earth Niobium Tantalum Abitibi Greenstone-belt Trace-element Systematics Nd-isotope Heterogeneity Southern West Greenland Plume-arc Interaction Superior Province Geological Samples Northwest-territories Lithospheric Mantle Supracrustal Belt 260301 Geochronology and Isotope Geochemistry C1 780104 Earth sciences |
spellingShingle |
Geosciences Multidisciplinary Earth Niobium Tantalum Abitibi Greenstone-belt Trace-element Systematics Nd-isotope Heterogeneity Southern West Greenland Plume-arc Interaction Superior Province Geological Samples Northwest-territories Lithospheric Mantle Supracrustal Belt 260301 Geochronology and Isotope Geochemistry C1 780104 Earth sciences Kamber, BS Greig, A Schoenberg, R Collerson, KD A refined solution to Earth's hidden niobium: implications for evolution of continental crust and mode of core formation |
topic_facet |
Geosciences Multidisciplinary Earth Niobium Tantalum Abitibi Greenstone-belt Trace-element Systematics Nd-isotope Heterogeneity Southern West Greenland Plume-arc Interaction Superior Province Geological Samples Northwest-territories Lithospheric Mantle Supracrustal Belt 260301 Geochronology and Isotope Geochemistry C1 780104 Earth sciences |
description |
New high-precision niobium (Nb) and tantalum (Ta) concentration data are presented for early Archaean metabasalts, metabasaltic komatiites and their erosion products (mafic metapelites) from SW Greenland and the Acasta gneiss complex, Canada. Individual datasets consistently show sub-chondritic Nb/Ta ratios averaging 15.1+/-11.6. This finding is discussed with regard to two competing models for the solution of the Nb-deficit that characterises the accessible Earth. Firstly, we test whether Nb could have sequestered into the core due to its slightly siderophile (or chalcophile) character under very reducing conditions, as recently proposed from experimental evidence. We demonstrate that troilite inclusions of the Canyon Diablo iron meteorite have Nb and V concentrations in excess of typical chondrites but that the metal phase of the Grant, Toluca and Canyon Diablo iron meteorites do not have significant concentrations of these lithophile elements. We find that if the entire accessible Earth Nb-deficit were explained by Nb in the core, only ca. 17% of the mantle could be depleted and that by 3.7 Ga, continental crust would have already achieved ca. 50% of its present mass. Nb/Ta systematics of late Archaean metabasalts compiled from the literature would further require that by 2.5 Ga, 90% of the present mass of continental crust was already in existence. As an alternative to this explanation, we propose that the average Nb/Ta ratio (15.1+/-11.6) of Earth's oldest mafic rocks is a valid approximation for bulk silicate Earth. This would require that ca. 13% of the terrestrial Nb resided in the Ta-free core. Since the partitioning of Nb between silicate and metal melts depends largely on oxygen fugacity and pressure, this finding could mean that metal/silicate segregation did not occur at the base of a deep magma ocean or that the early mantle was slightly less reducing than generally assumed. A bulk silicate Earth Nb/Ta ratio of 15.1 allows for depletion of up to 40% of the total mantle. This could indicate that in addition to the upper mantle, a portion of the lower mantle is depleted also, or if only the upper mantle were depleted, an additional hidden high Nb/Ta reservoir must exist. Comparison of Nb/Ta systematics between early and late Archaean metabasalts supports the latter idea and indicates deeply subducted high Nb/Ta eclogite slabs could reside in the mantle transition zone or the lower mantle. Accumulation of such slabs appears to have commenced between 2.5 and 2.0 Ga. Regardless of these complexities of terrestrial Nb/Ta systematics, it is shown that the depleted mantle Nb/Th ratio is a very robust proxy for the amount of extracted continental crust, because the temporal evolution of this ratio is dominated by Th-loss to the continents and not Nb-retention in the mantle. We present a new parameterisation of the continental crust volume versus age curve that specifically explores the possibility of lithophile element loss to the core and storage of eclogite slabs in the transition zone. (C) 2003 Elsevier Science B.V. All rights reserved. |
author2 |
K A Eriksson A Kroener |
format |
Article in Journal/Newspaper |
author |
Kamber, BS Greig, A Schoenberg, R Collerson, KD |
author_facet |
Kamber, BS Greig, A Schoenberg, R Collerson, KD |
author_sort |
Kamber, BS |
title |
A refined solution to Earth's hidden niobium: implications for evolution of continental crust and mode of core formation |
title_short |
A refined solution to Earth's hidden niobium: implications for evolution of continental crust and mode of core formation |
title_full |
A refined solution to Earth's hidden niobium: implications for evolution of continental crust and mode of core formation |
title_fullStr |
A refined solution to Earth's hidden niobium: implications for evolution of continental crust and mode of core formation |
title_full_unstemmed |
A refined solution to Earth's hidden niobium: implications for evolution of continental crust and mode of core formation |
title_sort |
refined solution to earth's hidden niobium: implications for evolution of continental crust and mode of core formation |
publisher |
Elsevier BV |
publishDate |
2003 |
url |
https://espace.library.uq.edu.au/view/UQ:66654 |
long_lat |
ENVELOPE(-57.289,-57.289,-63.799,-63.799) |
geographic |
Canada Diablo Greenland Northwest Territories |
geographic_facet |
Canada Diablo Greenland Northwest Territories |
genre |
Greenland Northwest Territories |
genre_facet |
Greenland Northwest Territories |
op_relation |
doi:10.1016/S0301-9268(03)00100-1 issn:0301-9268 |
op_doi |
https://doi.org/10.1016/S0301-9268(03)00100-1 |
container_title |
Precambrian Research |
container_volume |
126 |
container_issue |
3-4 |
container_start_page |
289 |
op_container_end_page |
308 |
_version_ |
1766019506794135552 |
spelling |
ftunivqespace:oai:espace.library.uq.edu.au:UQ:66654 2023-05-15T16:29:48+02:00 A refined solution to Earth's hidden niobium: implications for evolution of continental crust and mode of core formation Kamber, BS Greig, A Schoenberg, R Collerson, KD K A Eriksson A Kroener 2003-01-01 https://espace.library.uq.edu.au/view/UQ:66654 eng eng Elsevier BV doi:10.1016/S0301-9268(03)00100-1 issn:0301-9268 Geosciences Multidisciplinary Earth Niobium Tantalum Abitibi Greenstone-belt Trace-element Systematics Nd-isotope Heterogeneity Southern West Greenland Plume-arc Interaction Superior Province Geological Samples Northwest-territories Lithospheric Mantle Supracrustal Belt 260301 Geochronology and Isotope Geochemistry C1 780104 Earth sciences Journal Article 2003 ftunivqespace https://doi.org/10.1016/S0301-9268(03)00100-1 2020-10-26T23:32:22Z New high-precision niobium (Nb) and tantalum (Ta) concentration data are presented for early Archaean metabasalts, metabasaltic komatiites and their erosion products (mafic metapelites) from SW Greenland and the Acasta gneiss complex, Canada. Individual datasets consistently show sub-chondritic Nb/Ta ratios averaging 15.1+/-11.6. This finding is discussed with regard to two competing models for the solution of the Nb-deficit that characterises the accessible Earth. Firstly, we test whether Nb could have sequestered into the core due to its slightly siderophile (or chalcophile) character under very reducing conditions, as recently proposed from experimental evidence. We demonstrate that troilite inclusions of the Canyon Diablo iron meteorite have Nb and V concentrations in excess of typical chondrites but that the metal phase of the Grant, Toluca and Canyon Diablo iron meteorites do not have significant concentrations of these lithophile elements. We find that if the entire accessible Earth Nb-deficit were explained by Nb in the core, only ca. 17% of the mantle could be depleted and that by 3.7 Ga, continental crust would have already achieved ca. 50% of its present mass. Nb/Ta systematics of late Archaean metabasalts compiled from the literature would further require that by 2.5 Ga, 90% of the present mass of continental crust was already in existence. As an alternative to this explanation, we propose that the average Nb/Ta ratio (15.1+/-11.6) of Earth's oldest mafic rocks is a valid approximation for bulk silicate Earth. This would require that ca. 13% of the terrestrial Nb resided in the Ta-free core. Since the partitioning of Nb between silicate and metal melts depends largely on oxygen fugacity and pressure, this finding could mean that metal/silicate segregation did not occur at the base of a deep magma ocean or that the early mantle was slightly less reducing than generally assumed. A bulk silicate Earth Nb/Ta ratio of 15.1 allows for depletion of up to 40% of the total mantle. This could indicate that in addition to the upper mantle, a portion of the lower mantle is depleted also, or if only the upper mantle were depleted, an additional hidden high Nb/Ta reservoir must exist. Comparison of Nb/Ta systematics between early and late Archaean metabasalts supports the latter idea and indicates deeply subducted high Nb/Ta eclogite slabs could reside in the mantle transition zone or the lower mantle. Accumulation of such slabs appears to have commenced between 2.5 and 2.0 Ga. Regardless of these complexities of terrestrial Nb/Ta systematics, it is shown that the depleted mantle Nb/Th ratio is a very robust proxy for the amount of extracted continental crust, because the temporal evolution of this ratio is dominated by Th-loss to the continents and not Nb-retention in the mantle. We present a new parameterisation of the continental crust volume versus age curve that specifically explores the possibility of lithophile element loss to the core and storage of eclogite slabs in the transition zone. (C) 2003 Elsevier Science B.V. All rights reserved. Article in Journal/Newspaper Greenland Northwest Territories The University of Queensland: UQ eSpace Canada Diablo ENVELOPE(-57.289,-57.289,-63.799,-63.799) Greenland Northwest Territories Precambrian Research 126 3-4 289 308 |