Open AccessCombined 147,146 Sm‐ 143,142 Nd constraints on the longevity and residence time of early terrestrial crust
Author(s) -
Roth Antoine S. G.,
Bourdon Bernard,
Mojzsis Stephen J.,
Rudge John F.,
Guitreau Martin,
BlichertToft Janne
Publication year - 2014
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1002/2014gc005313
Subject(s) - zircon , felsic , geology , archean , geochemistry , hadean , crust , mafic , gneiss , continental crust , terrane , mantle (geology) , radiogenic nuclide , metamorphic rock , paleontology , tectonics
Primordial silicate differentiation controlled the composition of Earth's oldest crust. Inherited 142 Nd anomalies in Archean rocks are vestiges of the mantle‐crust differentiation before ca. 4300 Ma. Here we report new whole‐rock 147,146 Sm‐ 143,142 Nd data for the Acasta Gneiss Complex (AGC; Northwest Territories, Canada). Our 147 Sm‐ 143 Nd data combined with literature data define an age of 3371 ± 141 Ma (2 SD) and yield an initial ε 143 Nd 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 142 Nd deficits of −9.6 ± 4.8 ppm (2 SD) relative to the modern Earth. The discrepancy between 142 Nd anomalies and a mid‐Archean 147 Sm‐ 143 Nd 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 142 Nd/ 144 Nd identical to the modern Earth and a mafic component with 142 Nd/ 144 Nd 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 142 Nd evolution in the mantle and does not require slow mantle stirring times nor modification of its convective mode.