Neodymium and lead isotope evidence for enriched early Archaean crust in North America

Abstract

THERE has been considerable debate as to whether isotopically enriched continental crust was an important geochemical reservoir in earliest Archaean times. Fundamental questions such as the volume of the early crust, its composition and the effect of its formation and/or recycling on mantle geochemistry, have still to be answered. The most direct clues to the nature of the earliest crust and mantle are obtained from the isotope systematics of old rocks^1-5. Here we report neodymium and lead isotope measurements and uranium–lead zircon geochronology from Archaean gneisses of the Slave Province, Northwest Territories, Canada. The gneisses contain zircons with cores older than 3.842 Gyr. One sample of tonalitic gneiss has a Nd chondritic model age of 4.1 Gyr and an ε_Nd (3.7 Gyr) of −4.8. This is the oldest reported chondritic model age for a terrestrial sample and provides evidence for strongly enriched pre-3.8-Gyr crust, a reservoir complementary to the depleted mantle already in existence by 3.8 Gyr before present^2,6.