The combined Hf and Nd isotope evolution of the depleted mantle requires Hadean continental formation
Meng Guo, Jun Korenaga
Abstract
The onset and rates of continental growth are first-order indicators of early Earth dynamics, and whether substantial crust existed in the Hadean or much later has long been debated. Here, we present a theoretical analysis of published Hf and Nd isotopic data representing the depleted mantle and demonstrate that continental growth must have started in the early Hadean. Whereas the traditional interpretation of depleted mantle signatures in crustal rocks assumes unrealistic instantaneous mantle mixing, our modeling incorporates the effect of a finite mixing time over which these signatures are recorded in rocks produced through mantle melting. This effect is shown to delay, by as much as 0.65 to 0.75 billion years, the appearance of the earliest depleted mantle signatures in continental crust. Our results suggest that published observations of εHf, ε 143 Nd, and μ 142 Nd require Hadean growth of continental crust, with a minimum of 50% of today’s continental volume already existing by the end of Hadean.