Letter | Published:

142Nd evidence for an enriched Hadean reservoir in cratonic roots

Nature volume 459, pages 11181121 (25 June 2009) | Download Citation

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Abstract

The isotope 146Sm undergoes α-decay to 142Nd, with a half-life of 103 million years. Measurable variations in the 142Nd/144Nd values of rocks resulting from Sm–Nd fractionation could therefore only have been produced within about 400 million years of the Solar System’s formation (that is, when 146Sm was extant). The 142Nd/144Nd compositions of terrestrial rocks1,2,3,4,5,6,7 are accordingly a sensitive monitor of the main silicate differentiation events that took place in the early Earth. High 142Nd/144Nd values measured in some Archaean rocks from Greenland1,2,3,4,5 hint at the existence of an early incompatible-element-depleted mantle. Here we present measurements of low 142Nd/144Nd values in 1.48-gigayear-(Gyr)-old lithospheric mantle-derived alkaline rocks from the Khariar nepheline syenite complex in southeastern India8. These data suggest that a reservoir that was relatively enriched in incompatible elements formed at least 4.2 Gyr ago and traces of its isotopic signature persisted within the lithospheric root of the Bastar craton until at least 1.48 Gyr ago. These low 142Nd/144Nd compositions may represent a diluted signature of a Hadean (4 to 4.57 Gyr ago) enriched reservoir that is characterized by even lower values. That no evidence of the early depleted mantle has been observed in rocks younger than 3.6 Gyr (refs 3, 4, 7) implies that such domains had effectively mixed back into the convecting mantle by then. In contrast, some early enriched components apparently escaped this fate. Thus, the mantle sampled by magmatism since 3.6 Gyr ago may be biased towards a depleted composition that would be balanced by relatively more enriched reservoirs that are ‘hidden’ in Hadean crust6, the D′′ layer9,10,11 of the lowermost mantle or, as we propose here, also within the roots of old cratons.

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Acknowledgements

D.U. thanks A. Bhattacharya and M. Raith for help during field work and sample collection. This project was funded by grants from the Deutsche Forschungsgemeinschaft (ME1717).

Author Contributions D.U. collected the samples, processed them through ion exchange chromatography and measured the isotopic compositions. All authors contributed to the writing and editing of the manuscript.

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  1. Zentrallabor für Geochronologie, Institut für Mineralogie, Universität Münster, Corrensstrasse 24, 48149 Münster, Germany

    • Dewashish Upadhyay
    • , Erik E. Scherer
    •  & Klaus Mezger

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Correspondence to Dewashish Upadhyay.

Supplementary information

PDF files

  1. 1.

    Supplementary Information

    This file contains Supplementary Methods and Data, Supplementary Figures S1-S4 with Legends and Supplementary References.

Excel files

  1. 1.

    Supplementary Table S1

    This file shows Sm-Nd concentrations and Nd isotope composition of standards and samples.

  2. 2.

    Supplementary Table S2

    This file shows mass balance constraints for the 142Nd/144Nd compositions of present day terrestrial geochemical reservoirs.

  3. 3.

    Supplementary Table S3

    This file shows the estimated mass of the EER for gabbro-like and KREEP-like Nd concentrations.

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DOI

https://doi.org/10.1038/nature08089

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