Combined147,146Sm‐143,142Nd constraints on the longevity and residence time of early terrestrial crust

Abstract

AbstractPrimordial silicate differentiation controlled the composition of Earth's oldest crust. Inherited 142Nd anomalies in Archean rocks are vestiges of the mantle‐crust differentiation before ca. 4300 Ma. Here we report new whole‐rock 147,146Sm‐143,142Nd data for the Acasta Gneiss Complex (AGC; Northwest Territories, Canada). Our 147Sm‐143Nd data combined with literature data define an age of 3371 ± 141 Ma (2 SD) and yield an initial ε143Nd of −5.6 ± 2.1. These results are at odds with the Acasta zircon U‐Pb record, which comprises emplacement ages of 3920–3960 Ma. Ten of our thirteen samples show 142Nd deficits of −9.6 ± 4.8 ppm (2 SD) relative to the modern Earth. The discrepancy between 142Nd anomalies and a mid‐Archean 147Sm‐143Nd age can be reconciled with Nd isotope reequilibration of the AGC during metamorphic perturbations at ca. 3400 Ma. A model age of ca. 4310 Ma is derived for the early enrichment of the Acasta source. Two compositional end‐members can be identified: a felsic component with 142Nd/144Nd identical to the modern Earth and a mafic component with 142Nd/144Nd as low as −14.1 ppm. The ca. 4310 Ma AGC source is ∼200 Myr younger than those estimated for Nuvvuagittuq (northern Québec) and Isua (Itsaq Gneiss Complex, West Greenland). The AGC does not have the same decoupled Nd‐Hf isotope systematics as these other two terranes, which have been attributed to the crystallization of an early magma ocean. The Acasta signature rather is ascribed to the formation of Hadean crust that was preserved for several hundred Myr. Its longevity can be linked to 142Nd evolution in the mantle and does not require slow mantle stirring times nor modification of its convective mode.