Combined 147,146 Sm- 143,142 Nd constraints on the longevity and residence time of early terrestrial crust

Antoine Roth , Bernard Bourdon (1) , Stephen Mojzsis , John Rudge , Martin Guitreau (2) , Janne Blichert-Toft (3)

1 Institute of Geochemistry and Petrology [ETH Zürich]
2 LGL-TPE - Laboratoire de Géologie de Lyon - Terre, Planètes, Environnement
3 LST - Laboratoire de Sciences de la Terre

Primordial silicate differentiation controlled the composition of Earth's oldest crust. Inherited Nd-142 anomalies in Archean rocks are vestiges of the mantle-crust differentiation before ca. 4300 Ma. Here we report new whole-rock Sm-147,Sm-146-Nd-143,Nd-142 data for the Acasta Gneiss Complex (AGC; Northwest Territories, Canada). Our Sm-147-Nd-143 data combined with literature data define an age of 3371 +/- 141 Ma (2 SD) and yield an initial epsilon Nd-143 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 Nd-142 deficits of -9.6 +/- 4.8 ppm (2 SD) relative to the modern Earth. The discrepancy between Nd-142 anomalies and a mid-Archean Sm-147-Nd-143 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 Nd-142/Nd-144 identical to the modern Earth and a mafic component with Nd-142/Nd-144 as low as -14.1 ppm. The ca. 4310 Ma AGC source is similar to 200 Myr younger than those estimated for Nuvvuagittuq (northern Quebec) 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 Nd-142 evolution in the mantle and does not require slow mantle stirring times nor modification of its convective mode.

Domaines: Géochimie

Antoine Roth, Bernard Bourdon, Stephen Mojzsis, John Rudge, Martin Guitreau, et al.. Combined 147,146 Sm- 143,142 Nd constraints on the longevity and residence time of early terrestrial crust. Geochemistry, Geophysics, Geosystems, 2014, 15 (6), pp.2329-2345. ⟨10.1002/2014GC005313⟩. ⟨hal-02110388⟩