Isotopic evidence for crust-mantle evolution with emphasis on the Canadian Shield

G. R. Tilton, Sung Tack Kwon

Research output: Contribution to journalArticle

34 Citations (Scopus)

Abstract

Nd, Sr and Pb isotope data from carbonatites and related silicate rocks spanning an age range from 0.1 to 2.7 Ga in the Grenville and Superior provinces, Canadian Shield, trace the secular geochemical evolution of large ion lithophile element-depleted mantle sources beneath those provinces over the past 2.2 Ga. Sr data from 2.67-Ga-old syenites from the Abitibi, Quetico and Wabigoon belts in the Superior Province do not indicate depleted mantle sources, although ε{lunate}Nd for the same rocks averages +1. Initial Pb ratios in the syenites yield single-stage model ages that agree closely with their radiometric ages, while the model ages for the younger complexes are substantially younger than their radiometric ages. This suggests either that Pb in the 2.67-Ga complexes was not derived from aged depleted mantle sources, or that any recycled crustal component with high U/Pb had previously experienced only a short crustal residence time. The combined Pb and Sr data suggest that the depleted mantle beneath the Superior Province sampled by the complexes was initiated ∼3 Ga ago, in agreement with previously published results. Available data on carbonatites from the Fennoscandian Shield parallel the Canadian Shield isotope evolution pattern, although data from the southern hemisphere show considerable divergence, indicating regional variations. However, isotope evolution data from Precambrian marine precipitates suggest that major contributions of Sr to the oceans from sialic crust commenced ∼3 Ga ago. Assuming a complementary relationship between sialic crust and depleted mantle, the Canadian and European Sr data agree with average world-wide evolution of depleted mantle, and therefore appear to have more than regional significance. Models to reconcile the differences between the Nd and Sr results are considered. With present information several models are viable, some of which involve the first appearance of major volumes of depleted mantle around 3.0 Ga due to formation of sialic continental crust. The contrast between the Sr and Nd data may be due to differences in the way in which the bulk silicate Earth isotope evolution models for Nd compared to those for Sr and Pb are derived. Until the cause of the differences between Nd and the Sr and Pb data are better understood the presence of substantial volumes of sialic continental crust prior to ∼3.0 Ga ago is open to question.

Original languageEnglish
Pages (from-to)149-163
Number of pages15
JournalChemical Geology
Volume83
Issue number3-4
DOIs
Publication statusPublished - 1990 Jun 25

Fingerprint

shield
Isotopes
crust
mantle
Silicates
mantle source
isotope
Rocks
continental crust
Heavy ions
silicate
Precipitates
Earth (planet)
rock
Southern Hemisphere
residence time
Precambrian
divergence
province
ion

All Science Journal Classification (ASJC) codes

  • Geology
  • Geochemistry and Petrology

Cite this

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title = "Isotopic evidence for crust-mantle evolution with emphasis on the Canadian Shield",
abstract = "Nd, Sr and Pb isotope data from carbonatites and related silicate rocks spanning an age range from 0.1 to 2.7 Ga in the Grenville and Superior provinces, Canadian Shield, trace the secular geochemical evolution of large ion lithophile element-depleted mantle sources beneath those provinces over the past 2.2 Ga. Sr data from 2.67-Ga-old syenites from the Abitibi, Quetico and Wabigoon belts in the Superior Province do not indicate depleted mantle sources, although ε{lunate}Nd for the same rocks averages +1. Initial Pb ratios in the syenites yield single-stage model ages that agree closely with their radiometric ages, while the model ages for the younger complexes are substantially younger than their radiometric ages. This suggests either that Pb in the 2.67-Ga complexes was not derived from aged depleted mantle sources, or that any recycled crustal component with high U/Pb had previously experienced only a short crustal residence time. The combined Pb and Sr data suggest that the depleted mantle beneath the Superior Province sampled by the complexes was initiated ∼3 Ga ago, in agreement with previously published results. Available data on carbonatites from the Fennoscandian Shield parallel the Canadian Shield isotope evolution pattern, although data from the southern hemisphere show considerable divergence, indicating regional variations. However, isotope evolution data from Precambrian marine precipitates suggest that major contributions of Sr to the oceans from sialic crust commenced ∼3 Ga ago. Assuming a complementary relationship between sialic crust and depleted mantle, the Canadian and European Sr data agree with average world-wide evolution of depleted mantle, and therefore appear to have more than regional significance. Models to reconcile the differences between the Nd and Sr results are considered. With present information several models are viable, some of which involve the first appearance of major volumes of depleted mantle around 3.0 Ga due to formation of sialic continental crust. The contrast between the Sr and Nd data may be due to differences in the way in which the bulk silicate Earth isotope evolution models for Nd compared to those for Sr and Pb are derived. Until the cause of the differences between Nd and the Sr and Pb data are better understood the presence of substantial volumes of sialic continental crust prior to ∼3.0 Ga ago is open to question.",
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Isotopic evidence for crust-mantle evolution with emphasis on the Canadian Shield. / Tilton, G. R.; Kwon, Sung Tack.

In: Chemical Geology, Vol. 83, No. 3-4, 25.06.1990, p. 149-163.

Research output: Contribution to journalArticle

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N2 - Nd, Sr and Pb isotope data from carbonatites and related silicate rocks spanning an age range from 0.1 to 2.7 Ga in the Grenville and Superior provinces, Canadian Shield, trace the secular geochemical evolution of large ion lithophile element-depleted mantle sources beneath those provinces over the past 2.2 Ga. Sr data from 2.67-Ga-old syenites from the Abitibi, Quetico and Wabigoon belts in the Superior Province do not indicate depleted mantle sources, although ε{lunate}Nd for the same rocks averages +1. Initial Pb ratios in the syenites yield single-stage model ages that agree closely with their radiometric ages, while the model ages for the younger complexes are substantially younger than their radiometric ages. This suggests either that Pb in the 2.67-Ga complexes was not derived from aged depleted mantle sources, or that any recycled crustal component with high U/Pb had previously experienced only a short crustal residence time. The combined Pb and Sr data suggest that the depleted mantle beneath the Superior Province sampled by the complexes was initiated ∼3 Ga ago, in agreement with previously published results. Available data on carbonatites from the Fennoscandian Shield parallel the Canadian Shield isotope evolution pattern, although data from the southern hemisphere show considerable divergence, indicating regional variations. However, isotope evolution data from Precambrian marine precipitates suggest that major contributions of Sr to the oceans from sialic crust commenced ∼3 Ga ago. Assuming a complementary relationship between sialic crust and depleted mantle, the Canadian and European Sr data agree with average world-wide evolution of depleted mantle, and therefore appear to have more than regional significance. Models to reconcile the differences between the Nd and Sr results are considered. With present information several models are viable, some of which involve the first appearance of major volumes of depleted mantle around 3.0 Ga due to formation of sialic continental crust. The contrast between the Sr and Nd data may be due to differences in the way in which the bulk silicate Earth isotope evolution models for Nd compared to those for Sr and Pb are derived. Until the cause of the differences between Nd and the Sr and Pb data are better understood the presence of substantial volumes of sialic continental crust prior to ∼3.0 Ga ago is open to question.

AB - Nd, Sr and Pb isotope data from carbonatites and related silicate rocks spanning an age range from 0.1 to 2.7 Ga in the Grenville and Superior provinces, Canadian Shield, trace the secular geochemical evolution of large ion lithophile element-depleted mantle sources beneath those provinces over the past 2.2 Ga. Sr data from 2.67-Ga-old syenites from the Abitibi, Quetico and Wabigoon belts in the Superior Province do not indicate depleted mantle sources, although ε{lunate}Nd for the same rocks averages +1. Initial Pb ratios in the syenites yield single-stage model ages that agree closely with their radiometric ages, while the model ages for the younger complexes are substantially younger than their radiometric ages. This suggests either that Pb in the 2.67-Ga complexes was not derived from aged depleted mantle sources, or that any recycled crustal component with high U/Pb had previously experienced only a short crustal residence time. The combined Pb and Sr data suggest that the depleted mantle beneath the Superior Province sampled by the complexes was initiated ∼3 Ga ago, in agreement with previously published results. Available data on carbonatites from the Fennoscandian Shield parallel the Canadian Shield isotope evolution pattern, although data from the southern hemisphere show considerable divergence, indicating regional variations. However, isotope evolution data from Precambrian marine precipitates suggest that major contributions of Sr to the oceans from sialic crust commenced ∼3 Ga ago. Assuming a complementary relationship between sialic crust and depleted mantle, the Canadian and European Sr data agree with average world-wide evolution of depleted mantle, and therefore appear to have more than regional significance. Models to reconcile the differences between the Nd and Sr results are considered. With present information several models are viable, some of which involve the first appearance of major volumes of depleted mantle around 3.0 Ga due to formation of sialic continental crust. The contrast between the Sr and Nd data may be due to differences in the way in which the bulk silicate Earth isotope evolution models for Nd compared to those for Sr and Pb are derived. Until the cause of the differences between Nd and the Sr and Pb data are better understood the presence of substantial volumes of sialic continental crust prior to ∼3.0 Ga ago is open to question.

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