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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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.
This file shows Sm-Nd concentrations and Nd isotope composition of standards and samples.
This file shows mass balance constraints for the 142Nd/144Nd compositions of present day terrestrial geochemical reservoirs.
This file shows the estimated mass of the EER for gabbro-like and KREEP-like Nd concentrations.
About this article
Nature Geoscience (2010)