1. Department of Earth and Space Sciences and IGPP Center for Astrobiology, University of California Los Angeles, Los Angeles, California 90095-1567, USA
2. Department of Applied Geology, Curtin University of Technology, Perth, WA 6001, Australia
3. Present address: Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309-0399, USA.

Correspondence to: Stephen J. Mojzsis^1,2 Correspondence and requests for materials should be addressed to S.J.M. (e-mail: Email: Steve.Mojzsis@lasp.colorado.edu).

Granitoid gneisses and supracrustal rocks that are 3,800–4,000 Myr old are the oldest recognized exposures of continental crust¹. To obtain insight into conditions at the Earth's surface more than 4 Gyr ago requires the analysis of yet older rocks or their mineral remnants. Such an opportunity is presented by detrital zircons more than 4 Gyr old found within 3-Gyr-old quartzitic rocks in the Murchison District of Western Australia^{2, 3}. Here we report in situ U–Pb and oxygen isotope results for such zircons that place constraints on the age and composition of their sources and may therefore provide information about the nature of the Earth's early surface. We find that 3,910–4,280 Myr old zircons have oxygen isotope (¹⁸O) values ranging from 5.4 0.6 to 15.0 0.4. On the basis of these results, we postulate that the 4,300-Myr-old zircons formed from magmas containing a significant component of re-worked continental crust that formed in the presence of water near the Earth's surface. These data are therefore consistent with the presence of a hydrosphere interacting with the crust by 4,300 Myr ago.