Progress in the Application of Oxygen Isotopes in the Study of Petrogenesis

Since different types of rocks on the earth have different oxygen isotope compositions, they can be used to discuss the origins of various rocks and it has become a powerful tool for studying petrology. For example, determination on whether the granite was derived from metasedimentary or metavolcanic rocks based on its oxygen isotope compositions can be established. For a complex massif, if rocks of different stages have significantly different oxygen isotope compositions, the determination that they have been assimilated by other materials during evolution of magmas, because there should be no evident oxygen isotope fractionation (less than 0.3‰) during chemical differentiation of magmas from mafic to felsic composition can be made. Analytical techniques of oxygen isotope compositions include traditional BrF₅, laser BrF₅ and ion microprobe and they reflect the development from bulk analysis to microanalysis. Granites (rhyolites) and metamorphic rocks are used as examples to show that understandings of the origins of rocks have improved with the development of oxygen isotope composition analyses. For both granites (rhyolites) and metamorphic rocks, the ion microprobe in-situ analysis of oxygen isotope composition gain new insights into the origins of rocks. Based on traditional BrF₅ method, the Suzhou granite was suggested to have two different origins, a low-δ¹⁸O origin and a normal δ¹⁸O origin. However, δ¹⁸O values of magmatic zircons acquired by laser BrF₅ analysis are 4.92‰±0.26‰, confirming that the Suzhou granite has a low-δ¹⁸O magma origin that was derived from the crust. Similarly, the proposal drawn from traditional BrF₅ analysis on whole rock and rock-forming minerals cannot explain the intergranular and intraparticle oxygen isotope variations in the low-δ¹⁸O rhyolites from the Yellowstone plateau which can only be gained by the ion microprobe method. Based on laser BrF₅ analysis, the metamorphic rocks of the Sulu orogenic belt were considered to have acquired their extremely negative δ¹⁸O values during the formation of their protoliths in the Neoproterozoic. However, recent ion microprobe in-situ analysis of oxygen isotope compositions on zircon demonstrates that the Sulu metamorphic rocks acquired their extremely negative δ¹⁸O values during the ultrahigh pressure metamorphism in the Triassic. The study of distribution of oxygen isotopic composition in single gain is a new trend in the future.