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

Correspondence to: Dewashish Upadhyay¹ Correspondence and requests for materials should be addressed to D.U. (Email: upadhyay@uni-muenster.de).

The isotope ¹⁴⁶Sm undergoes -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 rocks^{1, 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 Greenland^{1, 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'' layer^{9, 10, 11} of the lowermost mantle or, as we propose here, also within the roots of old cratons.

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