4.4 billion years of crustal maturation: oxygen isotope ratios of magmatic zircon

Analysis of delta O-18 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 delta O-18 values higher than in primitive mantle magmas. This study summarizes data for delta O-18 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 delta O-18 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 delta O-18 of primitive mantle melts have remained constant (+/- 0.2 parts per thousand) for the past 4.4 billion years. The range and variability of delta O-18 in all Archean zircon samples is subdued (delta O-18(Zrc)=5-7.5 parts per thousand) ranging from values in high temperature equilibrium with the mantle (5.3 +/- 0.3 parts per thousand) to slightly higher, more evolved compositions (6.5-7.5 parts per thousand) including samples from: the Jack Hills (4.4-3.3 Ga), the Beartooth Mountains (4.0-2.9 Ga), Barberton (3.5-2.7 Ga), the Superior and Slave Provinces (3.0 to 2.7 Ga), and the Lewisian (2.7 Ga). No zircons from the Archean have been analyzed with magmatic delta O-18 above 7.5 parts per thousand. The mildly evolved, higher Archean values (6.5-7.5 parts per thousand) are interpreted to result from exchange of protoliths with surface waters at low temperature followed by melting or contamination to create mildly elevated magmas that host the zircons. During the Proterozoic, the range of delta O-18(Zrc) and the highest values gradually increased in a secular change that documents maturation of the crust. After similar to 1.5 Ga, high delta O-18 zircons (8 to > 10 parts per thousand) became common in many Proterozoic and Phanerozoic terranes reflecting delta O-18(whole rock) values from 9 to over 12 parts per thousand. The appearance of high delta O-18 magmas on Earth reflects nonuniformitarian changes in the composition of sediments, and rate and style of recycling of surface-derived material into magmas within the crust.