© 2006 Nature Publishing Group Episodic growth of the Gondwana supercontinent from hafnium and oxygen isotopes in zircon A. I. S. Kemp 1 , C. J. Hawkesworth 1 , B. A. Paterson 1 & P. D. Kinny 2 It is thought that continental crust existed as early as 150 million years after planetary accretion 1 , but assessing the rates and processes of subsequent crustal growth requires linking the apparently contradictory information from the igneous and sedi- mentary rock records. For example, the striking global peaks in juvenile igneous activity 2.7, 1.9 and 1.2 Gyr ago imply rapid crustal generation in response to the emplacement of mantle ‘super-plumes’, rather than by the continuous process of subduc- tion 2–4 . Yet uncertainties persist over whether these age peaks are artefacts of selective preservation 5 , and over how to reconcile episodic crust formation with the smooth crustal evolution curves inferred from neodymium isotope variations of sedimentary rocks 6,7 . Detrital zircons encapsulate a more representative record of igneous events than the exposed geology 1,8,9 and their hafnium isotope ratios reflect the time since the source of the parental magmas separated from the mantle. These ‘model’ ages are only meaningful if the host magma lacked a mixed or sedimentary source component 10 , but the latter can be diagnosed by oxygen isotopes, which are strongly fractionated by rock-hydrosphere interactions. Here we report the first study that integrates hafnium and oxygen isotopes, all measured in situ on the same, precisely dated detrital zircon grains. The data reveal that crust generation in part of Gondwana was limited to major pulses at 1.9 and 3.3 Gyr ago, and that the zircons crystallized during repeated reworking of crust formed at these times. The implication is that the mechanisms of crust formation differed from those of crustal differentiation in ancient orogenic belts. Palaeozoic sedimentary rocks that accumulated along the palaeo- Pacific margin of the former Gondwana supercontinent are ideal for testing punctuated crustal growth models. Their deposition postdates the major envisaged periods of rapid global crustal growth (2.7 and 1.9 Gyr ago; ref. 2), and their average crustal Sm/Nd ratios 7 and uniformity of detrital zircon age populations 11–13 testify to the efficient mixing of material eroded from large areas of continental crust. Detrital zircons in these rocks have ages that range to 3.5 Gyr, spanning most of Earth’s history. The two Ordovician greywackes (sandstones containing .15% clay) used in this study are from the eastern Australian segment of Gondwana, and were metamorphosed in the greenschist facies during formation of the Ordovician to Devonian Lachlan fold belt. We also analysed the inherited (pre- magmatic) zircon cores of two cordierite-bearing granite plutons that intruded 430 Myr ago, and those of an equivalent volcanic rock (Supplementary Table 1). The granitic magmas were generated from broadly metasedimentary precursors at mid-crustal depths 14 , but their abundant inherited zircons have the same range of ages 12,13,15 and isotope compositions as do detrital zircons in the exposed metasedimentary country rocks. Approximately 100 zircons from each rock were mounted in resin, polished to expose their centres and imaged by cathodoluminescence. Oscillatory or sector zonation is consistent with a magmatic para- genesis for the majority of these, although five irregularly banded grains with low Th/U ratios (,0.10) may represent metamorphic growth. The U–Pb, oxygen and Lu–Hf isotope compositions of each zircon were acquired by sequential, in situ analysis using three different instruments. The crystallization ages were first determined by SHRIMP (Sensitive High Resolution Ion MicroProbe) U–Pb isotope analysis at the Curtin University of Technology, Perth, under routine operating conditions 1 . Concordant grains, or those that were ,10% discordant for U–Pb isotopes, were then analysed for oxygen isotopes with a Cameca ims 1270 ion microprobe at the University of Edinburgh, the analysis site being directly adjacent to the SHRIMP pit. Last, Hf isotope ratios were measured from the same part of the zircon at the University of Bristol with a Finnigan Neptune multi-collector inductively coupled plasma- mass spectrometer (ICP-MS) and 193-nm ArF laser (Supplementary Methods). All data are given in Supplementary Tables 2, 3 and 4. Oxygen isotope ratios are expressed in the standard d 18 O notation, signifying deviation of the measured 18 O/ 16 O value from Vienna standard mean ocean water (VSMOW) in parts per thousand. Zircons in equilibrium with pristine mantle-derived melts have d 18 O values of 5.3 ^ 0.3‰ (ref. 16). Higher d 18 O values reflect a component of 18 O-enriched, supracrustal material in the magma from which the zircon precipitated, which could include sedimentary rock (10–40‰) or hydrothermally altered oceanic crust (10–20‰; ref. 17). Oxygen isotope ratios of igneous rocks are most sensitive to sedimentary input, as shown by the elevated d 18 O of metasedimentary-derived granites (9–15‰; ref. 18). Following ref. 19, we infer that zircons with d 18 O of less than 6.5‰ formed from melts that contained a minor to negligible sedimentary component. Zircons of this composition would crystallize from mafic melts with d 18 O values of ,7‰, or silicic magmas with d 18 O below 8‰, assuming equilibrium melt/zircon fractionation 20 . During this study we observed abrupt contrasts in d 18 O of nearly 6‰ between inherited zircon cores and their thin (,50 mm) magmatic over- growths. This accords with the empirically established low diffusion rate of oxygen in zircon under high-temperature conditions 21 , and suggests that the measured d 18 O of inherited zircons approximates the primary value. The 176 Hf/ 177 Hf data for detrital and inherited zircons with different d 18 O values are plotted as a function of their crystallization age in Fig. 1. Two striking features are evident. First, very few zircons have a Hf isotope composition that approaches that of the depleted mantle at the time of crystallization. The magmas from which most zircons formed were derived by melting pre-existing, rather than juvenile, crustal rocks. Second, zircons with d 18 O , 6.5‰ define two conspicuous linear arrays that intersect the depleted mantle growth curve near 1.9 and 3.3 Gyr ago. The low d 18 O precludes a significant LETTERS 1 Earth Sciences Department, University of Bristol, Bristol BS8 1RJ, UK. 2 Tectonics Special Research Centre, Department of Applied Geology, Curtin University of Technology, Perth 6845, Australia. Vol 439|2 February 2006|doi:10.1038/nature04505 580