The oxygen isotopic composition as an indicator of the genesis of 'Basaltic' corundum

摘要

This paper reports an oxygen isotopic study of corundums and associated minerals from the Yogo lamprophyres (Montana, US), plagioclase-corundum inclusions in alkali basalt (Tunkin depression, Russia), and from modern alluvium of Podgelbanochnyi alkaline basaltic volcano (Primorye, Russia). It is shown that all sapphires genetically related to mafic magmatic rocks have a similar oxygen isotopic composition (the variations of delta O-18 are within 2.5aEuro degrees) with most values plotting between +4.5 and +7.0aEuro degrees SMOW. The oxygen isotopic ratios in the associated minerals (olivines, pyroxenes, mica, and others) and host rocks are plotted in the same interval. This indicates that the sapphire crystallized during evolution of the parental mafic magma. However, there are xenogenic corundums, which were only transported by basaltic magma to the Earth's surface. They have a sharply distinct oxygen isotopic composition, which suggests their disequilibrium with the host lavas, and, correspondingly, a different genetic nature.