title: The elusive Hadean enriched reservoir revealed by 142Nd deficits in Isua Archaean rocks.
abstract: The first indisputable evidence for very early differentiation of the silicate Earth came from the extinct (146)Sm-(142)Nd chronometer. (142)Nd excesses measured in 3.7-billion-year (Gyr)-old rocks from Isua (southwest Greenland) relative to modern terrestrial samples imply their derivation from a depleted mantle formed in the Hadean eon (about 4,570-4,000 Gyr ago). As dictated by mass balance, the differentiation event responsible for the formation of the Isua early-depleted reservoir must also have formed a complementary enriched component. However, considerable efforts to find early-enriched mantle components in Isua have so far been unsuccessful. Here we show that the signature of the Hadean enriched reservoir, complementary to the depleted reservoir in Isua, is recorded in 3.4-Gyr-old mafic dykes intruding into the Early Archaean rocks. Five out of seven dykes carry (142)Nd deficits compared to the terrestrial Nd standard, with three samples yielding resolvable deficits down to -10.6 parts per million. The enriched component that we report here could have been a mantle reservoir that differentiated owing to the crystallization of a magma ocean, or could represent a mafic proto-crust that separated from the mantle more than 4.47 Gyr ago. Our results testify to the existence of an enriched component in the Hadean, and may suggest that the southwest Greenland mantle preserved early-formed heterogeneities until at least 3.4 Gyr ago.
authors.affiliation: Clermont Université, Université Blaise Pascal, Laboratoire Magmas et Volcans, CNRS UMR 6524, IRD R 163, BP 10448, 63000 Clermont-Ferrand, France. h.rizo@opgc.univ-bpclermont.fr
authors.forename: Hanika
authors.initials: H
authors.lastname: Rizo
authors.forename: Maud
authors.initials: M
authors.lastname: Boyet
authors.forename: Janne
authors.initials: J
authors.lastname: Blichert-Toft
authors.forename: Jonathan
authors.initials: J
authors.lastname: O'Neil
authors.forename: Minik T
authors.initials: MT
authors.lastname: Rosing
authors.forename: Jean-Louis
authors.initials: JL
authors.lastname: Paquette