AustralieFrV1, Main, Exploration, bibRecord, 00E127

Komatiites of the Onverwacht Group, S. Africa: REE geochemistry, Sm/Nd age and mantle evolution

Identifieur interne : 00E127 ( Main/Exploration ); précédent : 00E126; suivant : 00E128

Komatiites of the Onverwacht Group, S. Africa: REE geochemistry, Sm/Nd age and mantle evolution

Auteurs : Bor-Ming Jahn [France] ; G. Gruau [France] ; A. Y. Glikson [Australie]

Source :

Contributions to Mineralogy and Petrology [ 0010-7999 ] ; 1982-08-01.

RBID : ISTEX:D2969DA9D0DFCD992F2D3BC2AF0DDBB6FFD48D0B

Descripteurs français

Wicri :
- geographic : Finlande.
- topic : Géochimie, Essence.

English descriptors

Abstract

Abstract: Komatiites of the Tjakastad Subgroup of the Onverwacht Group (S. Africa) were dated by the Sm/Nd method. A whole-rock isochron yields an age of 3.56±0.24 (2σ) AE, with initial 143Nd/144Nd ratio of 0.50818±23 (2σ), corresponding to ɛ Nd(T)= + 1.9±4.5. This age is interpreted as the time of initial Onverwacht volcanism. This result agrees with earlier Sm/Nd data of Hamilton et al. (1979) and is consistent with the Rb-Sr result of Jahn and Shih (1974). Komatiites may be divided into 3 groups based on the typology of heavy REE distributions (Jahn and Gruau 1981). According to this scheme, the Onverwacht komatiites of the present study belong to two groups: the predominant Group II rocks showing (Gd/Yb)N≃1.4, CaO/Al2O3 = 1.33, Al2O3/TiO2≃10.6; and the subordinate Group III rocks with (Gd/Yb)N<1.0; CaO/Al2O3≃0.6 and A12O3/ TiO2≃40. This contrasting feature is best explained by garnet fractionation within the mantle sources. Younger komatiites (∼2.7 AE) from Finland, Canada, Rhodesia, and Australia have (Gd/Yb)N≃1.0, CaO/ Al2O3<1.1 and Al2O3/TiO2≃21 based on 58 analyses. These ratios are nearly chondritic or of the bulk earth value (Anders 1977). It appears that some late Archean komatiites are different in chemistry from many early Archean komatiites. This may imply that the upper mantle chemistry has evolved through Archean times. However, the age connotation of the chemical parameters, such as CaO/Al2O3, (Gd/Yb)N or Al2O3/TiO2 ratio has not been firmly established. The characteristic “high” CaO/Al2O3 or (Gd/Yb)N ratios in many Onverwacht Group rocks can also be explained as a result of local short-term mantle heterogeneity.

Url:

https://api.istex.fr/document/D2969DA9D0DFCD992F2D3BC2AF0DDBB6FFD48D0B/fulltext/pdf

DOI: 10.1007/BF00376732

Affiliations:

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Le document en format XML

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<term>Archaean</term>
<term>Archean</term>
<term>Archean komatiites</term>
<term>Archean mantle</term>
<term>Archean times</term>
<term>Arndt</term>
<term>Barberton</term>
<term>Basaltic</term>
<term>Basaltic komatiite</term>
<term>Basaltic komatiites</term>
<term>Cation plot</term>
<term>Chondritic</term>
<term>Chondritic ratios</term>
<term>Correlation coefficients</term>
<term>Data points</term>
<term>Distribution patterns</term>
<term>Early archean rocks</term>
<term>Earth planet</term>
<term>Enclave</term>
<term>Finland</term>
<term>Garnet</term>
<term>Garnet fractionation</term>
<term>Geochemistry</term>
<term>Geol</term>
<term>Glikson</term>
<term>Gneiss</term>
<term>Greenstone</term>
<term>Heterogeneity</term>
<term>High pressure</term>
<term>High ratios</term>
<term>Holenarsipur</term>
<term>Hree</term>
<term>Indian rocks</term>
<term>Isochron</term>
<term>Isotopic</term>
<term>Isotopic analyses</term>
<term>Isotopic data</term>
<term>Jahn</term>
<term>Komati</term>
<term>Komati formation</term>
<term>Komatiite</term>
<term>Komatiites</term>
<term>Komatiitic</term>
<term>Komatiitic rocks</term>
<term>Late archean komatiites</term>
<term>Late archean rocks</term>
<term>Lett</term>
<term>Local mantle heterogeneity</term>
<term>Lree</term>
<term>Mantle evolution</term>
<term>Mantle sources</term>
<term>Metamorphism</term>
<term>Microcrystalline</term>
<term>Nesbitt</term>
<term>Onverwacht</term>
<term>Onverwacht group</term>
<term>Onverwacht komatiites</term>
<term>Onverwacht rocks</term>
<term>Open symbols</term>
<term>Peridotitic</term>
<term>Peridotitic komatiite</term>
<term>Peridotitic komatiites</term>
<term>Petrogenesis</term>
<term>Petrogenetic models</term>
<term>Petrographic description</term>
<term>Petrol</term>
<term>Philos trans</term>
<term>Pilbara block</term>
<term>Plag</term>
<term>Present study</term>
<term>Rare earth elements</term>
<term>Rhodesia</term>
<term>Rock type</term>
<term>Rocks show</term>
<term>Sandspruit</term>
<term>Sauss</term>
<term>Secondary alteration</term>
<term>Shih</term>
<term>Sio2</term>
<term>Solid symbols</term>
<term>Southern africa</term>
<term>Subgroup</term>
<term>Theespruit</term>
<term>Tholeiitic</term>
<term>Tio2</term>
<term>Tjakastad subgroup</term>
<term>Trace element data</term>
<term>Trem</term>
<term>Upper mantle</term>
<term>Upper mantle chemistry</term>
<term>Upper mantle project</term>
<term>Viljoen</term>
<term>Volcanic rocks</term>
<term>Volcanism</term>
<term>Western australia</term>
<term>Younger komatiites</term>
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<term>Archaean</term>
<term>Archean</term>
<term>Archean komatiites</term>
<term>Archean mantle</term>
<term>Archean times</term>
<term>Arndt</term>
<term>Barberton</term>
<term>Basaltic</term>
<term>Basaltic komatiite</term>
<term>Basaltic komatiites</term>
<term>Cation plot</term>
<term>Chondritic</term>
<term>Chondritic ratios</term>
<term>Correlation coefficients</term>
<term>Data points</term>
<term>Distribution patterns</term>
<term>Early archean rocks</term>
<term>Earth planet</term>
<term>Enclave</term>
<term>Finland</term>
<term>Garnet</term>
<term>Garnet fractionation</term>
<term>Geochemistry</term>
<term>Geol</term>
<term>Glikson</term>
<term>Gneiss</term>
<term>Greenstone</term>
<term>Heterogeneity</term>
<term>High pressure</term>
<term>High ratios</term>
<term>Holenarsipur</term>
<term>Hree</term>
<term>Indian rocks</term>
<term>Isochron</term>
<term>Isotopic</term>
<term>Isotopic analyses</term>
<term>Isotopic data</term>
<term>Jahn</term>
<term>Komati</term>
<term>Komati formation</term>
<term>Komatiite</term>
<term>Komatiites</term>
<term>Komatiitic</term>
<term>Komatiitic rocks</term>
<term>Late archean komatiites</term>
<term>Late archean rocks</term>
<term>Lett</term>
<term>Local mantle heterogeneity</term>
<term>Lree</term>
<term>Mantle evolution</term>
<term>Mantle sources</term>
<term>Metamorphism</term>
<term>Microcrystalline</term>
<term>Nesbitt</term>
<term>Onverwacht</term>
<term>Onverwacht group</term>
<term>Onverwacht komatiites</term>
<term>Onverwacht rocks</term>
<term>Open symbols</term>
<term>Peridotitic</term>
<term>Peridotitic komatiite</term>
<term>Peridotitic komatiites</term>
<term>Petrogenesis</term>
<term>Petrogenetic models</term>
<term>Petrographic description</term>
<term>Petrol</term>
<term>Philos trans</term>
<term>Pilbara block</term>
<term>Plag</term>
<term>Present study</term>
<term>Rare earth elements</term>
<term>Rhodesia</term>
<term>Rock type</term>
<term>Rocks show</term>
<term>Sandspruit</term>
<term>Sauss</term>
<term>Secondary alteration</term>
<term>Shih</term>
<term>Sio2</term>
<term>Solid symbols</term>
<term>Southern africa</term>
<term>Subgroup</term>
<term>Theespruit</term>
<term>Tholeiitic</term>
<term>Tio2</term>
<term>Tjakastad subgroup</term>
<term>Trace element data</term>
<term>Trem</term>
<term>Upper mantle</term>
<term>Upper mantle chemistry</term>
<term>Upper mantle project</term>
<term>Viljoen</term>
<term>Volcanic rocks</term>
<term>Volcanism</term>
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<front><div type="abstract" xml:lang="en">Abstract: Komatiites of the Tjakastad Subgroup of the Onverwacht Group (S. Africa) were dated by the Sm/Nd method. A whole-rock isochron yields an age of 3.56±0.24 (2σ) AE, with initial 143Nd/144Nd ratio of 0.50818±23 (2σ), corresponding to ɛ Nd(T)= + 1.9±4.5. This age is interpreted as the time of initial Onverwacht volcanism. This result agrees with earlier Sm/Nd data of Hamilton et al. (1979) and is consistent with the Rb-Sr result of Jahn and Shih (1974). Komatiites may be divided into 3 groups based on the typology of heavy REE distributions (Jahn and Gruau 1981). According to this scheme, the Onverwacht komatiites of the present study belong to two groups: the predominant Group II rocks showing (Gd/Yb)N≃1.4, CaO/Al2O3 = 1.33, Al2O3/TiO2≃10.6; and the subordinate Group III rocks with (Gd/Yb)N<1.0; CaO/Al2O3≃0.6 and A12O3/ TiO2≃40. This contrasting feature is best explained by garnet fractionation within the mantle sources. Younger komatiites (∼2.7 AE) from Finland, Canada, Rhodesia, and Australia have (Gd/Yb)N≃1.0, CaO/ Al2O3<1.1 and Al2O3/TiO2≃21 based on 58 analyses. These ratios are nearly chondritic or of the bulk earth value (Anders 1977). It appears that some late Archean komatiites are different in chemistry from many early Archean komatiites. This may imply that the upper mantle chemistry has evolved through Archean times. However, the age connotation of the chemical parameters, such as CaO/Al2O3, (Gd/Yb)N or Al2O3/TiO2 ratio has not been firmly established. The characteristic “high” CaO/Al2O3 or (Gd/Yb)N ratios in many Onverwacht Group rocks can also be explained as a result of local short-term mantle heterogeneity.</div>
</front>
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Komatiites of the Onverwacht Group, S. Africa: REE geochemistry, Sm/Nd age and mantle evolution

Komatiites of the Onverwacht Group, S. Africa: REE geochemistry, Sm/Nd age and mantle evolution

Source :

Descripteurs français

English descriptors

Abstract

Links toward previous steps (curation, corpus...)

Le document en format XML

Pour manipuler ce document sous Unix (Dilib)

Pour mettre un lien sur cette page dans le réseau Wicri