Can oxygen isotopes in magmatic zircon be modified by metamorphism? A case study from the Eoarchean Dniester-Bug Series, Ukrainian Shield

• Zircon from a quartz dominated, metamorphic rock has U–Pb ages up to 3.8 Ga.
• The zircons include apparently unaltered crystals with preserved magmatic style oscillatory zonation.
• O isotope compositions are heavier than normally found in unaltered Archean zircon.
• This may indicate the existence of substantial evolved crust in the Eoarchean.
• We suggest an alternative interpretation that the heavy O signature may be metasomatically modified.

Zircon occurs as a minor constituent in most differentiated magmatic rocks. Its robustness to later modification means that its isotopic and chemical composition generally records conditions prevailing when it formed, and the systematic changes in the oxygen isotope record of zircon through geological time have been used to trace the temporal evolution of crust–mantle interaction and intra-crustal recycling. Here we present U–Pb, Hf, and oxygen isotopic compositions for high grade metamorphic Archean rocks from the Dniester-Bug Series, western Ukrainian Shield. Zircon from a quartz-dominated rock is up to 3.8 Ga old, and enriched in 18O compared to most previously reported values from Archean zircon. Similar values are recorded in zircon cores, which exhibit a variety of internal textures including magmatic-style oscillatory zonation, and rims. If this rock is metasedimentary and the isotope signatures in cores are primary, the zircon sources were characterized by heavier oxygen isotopic compositions than any known major area of Archean crust. Alternatively the O isotope compositions have been modified. We show that a large fraction of the analyzed zircon appear not to be modified by radiation damage, and speculate that O exchange may have taken place by diffusion during extreme metasomatic alteration of the host rock. The possibility that igneous-looking, apparently unaltered zircon may not preserve a primary oxygen isotope signature has implications for its use in the interpretation of crustal evolution, including early terrestrial geodynamics.