Nature of the Earth's earliest crust from hafnium isotopes in single detrital zircons

Continental crust forms from, and thus chemically depletes, the Earth's man tle. Evidence that the Earth's mantle was already chemically depleted by me lting before the formation of today's oldest surviving crust has been prese nted in the form of Sm-Nd isotope studies of 3.8-4.0 billion years old rock s from Greenland(1-5) and Canada(5-7). But this interpretation has been que stioned because of the possibility that subsequent perturbations may have r e-equilibrated the neodymium-isotope compositions of these rocks(8). Indepe ndent and more robust evidence for the origin of the earliest crust and dep letion of the Archaean mantle can potentially be provided by hafnium-isotop e compositions of zircon, a mineral whose age can be precisely determined b y U-Pb dating, and which can survive metamorphisms(4). But the amounts of h afnium in single zircon grains are too small for the isotopic composition t o be precisely analysed by conventional methods. Here we report hafnium-iso tope data, obtained using the new technique of multiple-collector plasma-so urce mass spectrometry(9), for 37 individual grains of the oldest known ter restrial zircons (from the Narryer Gneiss Complex, Australia, with U-Pb age s of up to 4.14 Gyr (refs 10-13)). We find that none of the grains has a de pleted mantle signature, but that many were derived from a source with a ha fnium-isotope composition similar to that of chondritic meteorites. Further more, more than half of the analysed grains seem to have formed by remeltin g of significantly older crust, indicating that crustal preservation and su bsequent reworking might have been important processes from earliest times.