Silicate Oxygen Isotope Geochemistry: History, Principles, Techniques, and Application to Petrological Problems

The importance of oxygen isotope geochemistry in studies of terrestrial and extra-terrestrial silicate rocks was recognized nearly sixtyeight years ago soon after the discovery of O-18 and O-17. As early as 1934, the significance of oxygen isotope variations in rocks and minerals was stressed by Russian geochemists who also pioneered the discipline of silicate oxygen isotope geochemistry. It is now known that processes involving isotopic interaction between rock and water, magmatic differentiation, and metamorphic recrystallization fractionate oxygen isotopes in the lithosphere. delta O-18 (the conventional notation for reporting O-18/O-16 ratios in rocks and minerals) is highest in sedimentary rocks (17 to 35 parts per thousand) and lowest in igneous rocks (4 to 12 parts per thousand). Metamorphic rocks have intermediate values. delta O-18 in mafic minerals (1 to 8 parts per thousand) is lower than in felsic minerals (8 to 16 parts per thousand). In igneous and metamorphic rocks, quartz is most enriched in O-18 (10 to 16 parts per thousand) and magnetite the least (1 to 2 parts per thousand). An important application of O-18/O-16 techniques is in geothermometry, where these are capable of elucidating several petrological processes.