Jack Hills, Western Australia

The Canadian Shield is one the most extensive ancient rock formations on Earth, and the rocks of the Acasta Gneiss, east of Great Slave Lake, at 4.03 billion years were thought to be the oldest rocks on Earth. Since then 4.276 billion year old detrital zircons from the Narryer Gneiss Terrane, of the Yilgarn Craton, Western Australia, at Jack Hills near Mt Narryer, were found. Detrital zircons have now been found at Jack Hills that date to 4.404 billion years ± 8 million years. Their oxygen isotope ratios and the presence of micro inclusions of SiO² indicate that they formed in molten granite in the presence of liquid water. This indicates that they are the oldest known instance of continental crust and liquid oceans.

Episodes, Journal of the International Union of Geological Sciences

Volume 24, No.2

June 2001

Review of the oldest (4400-3600 Ma) geological and mineralogical record: Glimpses of the beginning
Allen P. Nutman, Clark R. L. Friend, and Vickie C. Bennett

Known occurrences of rocks from the first billion years (?3550 Ma) form a minuscule ~10,000 Km2 of Earth's surface. The largest areas are in Greenland (Itsaq Gneiss Complex), Labrador and Western Australia, with smaller ones elsewhere in Greenland and in Antarctica, China and the Acasta area, Canada (containing the oldest-known terrestrial rocks at ~ 4030 Ma). 4000-4400 Ma detrital zircons in sediments at Mt Narryer and the Jack Hills, Western Australia are another important part of the first billion years record. The Itsaq Gneiss Complex (Greenland ) has the largest domains of least strain and migmatisation, with the most recognisable (amphibolite facies) sedimentary, volcanic and plutonic structures. These show a "normal" Earth by ~ 3800 Ma, with a hydrosphere, life and division of the lithosphere into granitic and mafic components of unremarkable composition. There is no apparent geological evidence for the effects of impacts. Isotopic and petrographic work on ancient Western Australian zircons implies granites and hydrosphere were present on Earth 4400-4200 million years ago. Isotopic studies of first billion years rocks and minerals show that juvenile granitoids were added repeatedly to continental crust in the Archaean, core formation occurred in the first 100 million years and also the mantle differentiated early on into chemically distinct domains.

Precambrian Research Volume 135, Issue 4, Pages 245-360 (15 December 2004)
The First Billion Years - Selected Papers Presented at the 13th V.M. Goldschmidt Conference, Kurashiki, Japan
Edited by S.J. Mojzsis

Internal zoning and U–Th–Pb chemistry of Jack Hills detrital zircons: a mineral record of early Archaean to Mesoproterozoic (4348–1576 Ma) magmatism • ARTICLE
Pages 251-279

Magmatic processes were important on the nascent Earth during the first 500 million years (Ma) after accretion, yet the causes and timing of this early magmatism are largely unconstrained, as no rocks from this period have been discovered. Rare >4000 Ma detrital zircons from Western Australia preserve the only direct geologic evidence of this early magmatism. To understand the genesis and history of these zircons, we present the results of a combined ion and electron microprobe, and SEM study of the age, Th–U chemistry, cathodoluminescence (CL) zoning patterns, and inclusions for a population of detrital zircons from Jack Hills, Western Australia, with 207Pb/206Pb ages ranging from 4348 to 1576 Ma. The majority of the zircons preserve primary growth features discernable by CL imaging, such as oscillatory and sector zoning, have Th/U ratios from 0.1 to 1.0, and several contain granitic mineral inclusions. Thus, aside from age they are largely indistinguishable from zircons produced in common felsic magmas. The Jack Hills zircons are therefore remnants of igneous rock-forming events that pre-date the rock record by up to 400 Ma. The 207Pb/206Pb age distribution pattern for zircons older than 3800 Ma from Western Australia suggests that early Archaean magmatism was punctuated, both in terms of high frequency events and conspicuous gaps. The variable age distributions within different rock units in the Jack Hills demonstrate that Early Archaean zircons were derived from multiple source rocks; samples from Eranondoo Hill contain up to 12% >4000 Ma zircons, suggesting either that the source rocks were nearby or represent a large terrane. Furthermore, younger 3700–3400 Ma rims on 4300–4000 Ma zircons are evidence that >4000 Ma crust survived long enough to participate in younger Archaean tectonic events in the Yilgarn Craton of Western Australia. Mesoproterozoic igneous zircons in a quartzite 50 m from Eranondoo Hill are attributed to either sedimentation or tectonic interleaving of younger sediments no earlier than 1576 Ma. This previously unrecognized Proterozoic (or younger) geologic history calls into question previous estimates of the age of the Jack Hills sediments and demonstrates the heterogeneous distribution of >4000 Ma grains within the belt.