The Acasta Gneisses: Remnant Of Earth’s Early Crust

Abstract

Abstract The 3.96-Ga Acasta gneisses from the Slave craton of northwestern Canada are the oldest rocks yet recognized on Earth. Uranium-lead ion-microprobe analyses of zircons separated from two samples, a banded tonalitic orthogneiss and a granitic orthogneiss, yield the same crystallization age of 3.96 Ga. The U-Pb zircon data from both samples define complex arrays on a concordia diagram and may be interpreted in terms of zircon crystallization at 3.96 Ga followed by an early Pb-loss event and crystallization of new zircon at ca. 3.6 Ga, followed by a Proterozoic or younger Pb-loss event. Neodymium and lead isotopic data suggest that crust older than 3.96 Ga was involved in the generation of these rocks. The tonalitic orthogneiss has a chondritic Nd model age of ca 4.1 Ga and an fNct(3.96 Ga) of -1.7. A mafic layer from the tonalitic orthogneiss and the granitic orthogneiss have fNct(3.96 Ga) of +0.7 and +0.8, respectively. The least radiogenic Pb isotopic compositions of feldspars from the orthogneisses are very radiogenic relative to early Archean model mantle compositions and have model ages considerably younger than their age. The feldspar Pb isotopic data indicate that these rocks were derived from a source region that had experienced a complex multistage history prior to 3.96 Ga and had a high U:Pb ratio during some or all of its evolution. Thus, the Acasta gneisses are the first example of an early Archean isotopically enriched reservoir. The U-Pb geochronology, Nd and Pb isotopic compositions, and geochemistry of the Acasta gneisses are consistent with the involvement of older crust in their formation. By implication, available Nd and Pb isotopic data from other Archean cratons worldwide may be interpreted as reflecting mixtures of older crust and mantle, rather than mantle heterogeneity alone. The 3.96-Ga Acasta gneisses are evidence that early, enriched, sialic crust was preserved, although isotopic and geochemical arguments suggest that it is not the sole enriched complement to the depleted mantle.