Neodymium isotope evidence for ultra-depleted mantle in the early Proterozoic

Abstract

THE episodic extraction of juvenile continental crust from the Earth's mantle over the past 4 Gyr has led to a progressive depletion of incompatible elements in the upper mantle1,2. A knowledge of the degree and uniformity of this mantle depletion throughout Earth history is important for understanding the growth of continents, the evolution of the crust-mantle system and the nature of mantle convection through time. Here we report initial 143Nd/144Nd ratios, for 1.8-Gyr-old mafic volcanics from the Harts Range meta-igneous complex of central Australia which are the highest yet reported for Proterozoic igneous rocks (ɛNd = +6.9 to +8.2 for the least contaminated samples). These ratios far exceed those proposed in models3–5 for the isotopic evolution of the depleted mantle at this time, and imply the existence of a mantle reservoir that had been highly depleted for at least 1 Gyr. This provides strong evidence that major periods of continental growth, such as that in the late Archaean, produced long-lived heterogeneities in the upper mantle.