American Mineralogist, Volume 92, pages 17721775, 2007 0003-004X/07/00101772$05.00/DOI: 10.2138/am.2007.2697 1772 INTRODUCTION Oxygen isotopic ratios are constant for primitive magmatic rocks from the Earths mantle, but can be highly fractionated by low-temperature processes on its surface. Thus, δ 18 O is a pow- erful tracer of recycled crust that has been buried and melted, especially if it interacted with liquid water at low temperatures before burial. The accessory mineral zircon found within these rocks can be dated using the U-Pb system and provides a temporal record of geochemical information (e.g., Hanchar and Hoskin 2003). Oxygen isotopes in zircon have been used to chronicle the maturation of Earths crust throughout geologic time (Valley et al. 2005), elucidate the origins of granite (Kemp et al. 2007), monitor differentiation of lunar crust (Nemchin et al. 2006b), and demonstrate the presence of liquid water on the surface of Earth in the earliest Archean (Wilde et al. 2001; Mojzsis et al. 2001; Cavosie et al. 2005). Although the concordance of U-Pb geochronology provides robust tests to evaluate post crystal- lization alteration, the oxygen isotope system contains no such safeguards and its resistance to chemical and physical altera- tion must be evaluated by other means. In particular, the rate of oxygen exchange by diffusion in zircon during high-grade metamorphism remains uncertain, and the retention of primary δ 18 O values is sometimes controversial. LETTER High-precision oxygen isotope analysis of picogram samples reveals 2 μm gradients and slow diffusion in zircon F. ZEB PAGE, 1, * T. USHIKUBO, 1 N.T. KITA, 1 L.R. RICIPUTI, 2 AND J.W. VALLEY 1 1 Department of Geology and Geophysics, University of WisconsinMadison, 1215 W. Dayton St., Madison, Wisconsin 53706, U.S.A. 2 Oak Ridge National Laboratory, P.O. Box 2008, MS6375, Oak Ridge, Tennessee 37831-6375, U.S.A. ABSTRACT Ion microprobe analysis with a sub-micrometer diameter spot reveals a sharp, 2 μm gradient in oxygen isotope ratio proving that oxygen diffusion in zircon is slow even under prolonged high-grade metamorphism. The data are consistent with an oxygen diffusion coefcient of 10 23.5–1 cm 2 /s. Fur- thermore, this gradient is found in a zircon that contains clear textural evidence of recrystallization in nearby regions. This nding shows that through careful textural and chemical analysis, primary information can be extracted from a zircon that has also undergone partial recrystallization. The oxygen isotope ratios found in zircon have been used to infer magmatic and pre-magmatic histories, including the presence of liquid water on the surface of earliest Earth. Recently, these interpretations have been questioned with the assertion that zircon may not retain its primary oxygen isotope signature through metamorphism. The slow diffusion conrmed by these results supports interpretations that assume preservation of magmatic compositions. Keywords: Ion microprobe, SIMS, zircon, diffusion, oxygen isotopes, stable isotopes, granulites facies, migmatites Several studies have shown that zircons preserve primary magmatic values of δ 18 O through episodes of metamorphism, magmatism, and hydrothermal alteration (Valley et al. 1994; Peck et al. 2003; Valley 2003). However, careful laboratory experiments to measure the rate of oxygen diffusion in zircon suggest that although zircon is extremely retentive of oxygen only in the absence of water; even a small amount of water greatly enhances the diffusion rate of oxygen (Watson and Cherniak 1997; Cherniak and Watson 2003). These experimental results have been broadly interpreted to suggest that igneous zircons that have undergone metamorphism do not retain any primary oxygen isotope information. In particular, the elevated δ 18 O found in the Early Archean zircons from the Jack Hills, Western Australia has been described as the product of hydrothermal alteration or granulite metamorphism, and therefore not a primary magmatic feature (Whitehouse and Kamber 2002; Nelson 2004; Hoskin 2005; Nemchin et al. 2006a). Analysis of U-Pb isotopes, trace elements, and δ 18 O by ion microprobe (Secondary Ion Mass Spectrometer, SIMS) in 2030 μm diameter domains of single zircons has recently become routine. At the 20 μm scale, multiple analyses of single zircon grains reveal heterogeneities in all geochemical systems of inter- est, often (but not always) correlated with growth zoning and recrystallization features visible in cathodoluminescence imaging (CL). For these reasons, Cavosie et al. (2006) correlated zoning to the location of analysis pits from different ion probe analyses of zircon, suggesting that even zircons with complicated histories of metamorphism and recrystallization may contain pristine domains that preserve primary compositions. * Present Address: Geology Department, Oberlin College, 52 W Lorain St., Oberlin, OH, 44074, U.S.A. E-mail: zeb.page@ oberlin.edu