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Analysis of δ¹⁸O in igneous zircons of known age traces the evolution of intracrustal recycling and crust-mantle interaction through time. This record is especially sensitive because oxygen isotope ratios of igneous rocks are strongly affected by incorporation of supracrustal materials into melts, which commonly have δ¹⁸O values higher than in primitive mantle magmas. This study summarizes data for δ¹⁸O in zircons that have been analyzed from 1,200 dated rocks ranging over 96% of the age of Earth. Uniformly primitive to mildly evolved magmatic δ¹⁸O values are found from the first half of Earth history, but much more varied values are seen for younger magmas. The similarity of values throughout the Archean, and comparison to the composition of the “modern” mantle indicate that δ¹⁸O of primitive mantle melts have remained constant (±0.2‰) for the past 4.4 billion years. The range and variability of δ¹⁸ …