Isotope Evolution of the Depleted Mantle

Abstract

The depleted mantle reservoir is that part of Earth's mantle from which crust has been extracted, leaving the remaining mantle depleted in incompatible elements. Knowing how and when it formed is essential for understanding the chemical evolution of Earth, including formation of continental crust. The best-constrained Hf isotope data presented here indicate that the mantle does not become significantly depleted until as late as 700 million years after Earth's accretion. This onset of mantle depletion coincides with the first appearance of substantial volumes of continental crust in the geological record. These data compel a revision to the reference depleted mantle parameters used in Hf isotope studies of planetary evolution. This new reference line follows chondritic evolution until 3.8 Ga and then describes a linear trajectory to a present-day depleted mid-ocean ridge basalt source mantle composition (εHf = +18). We infer that stabilization of continental crust only occurred in earnest on Earth after 3.8 Ga. ▪ Hf isotopes show that Earth's mantle does not become significantly depleted until 700 million years after planetary accretion. ▪ Most of Earth's oldest rocks formed from mantle sources that had radiogenic isotope compositions similar to those of chondritic meteorites. ▪ Isotope evidence shows that Hadean (>4.0-billion-year-old) crust was not essential for formation of younger crust in Archean terranes. ▪ Growth of Earth's continents only began in earnest after 3.8 Ga.