PERIDOTITE NODULES FROM THE UDACHNAYA KIMBERLITE PIPE: "NONMANTLE" OXYGEN ISOTOPE RATIOS IN GARNETS

Abstract

The peridotite nodules from kimberlites and basalts represent the lithospheric mantle and commonly show a very narrow range of δ18O = +5.5 ± 0.4‰. This is typical of spinel and garnet (low- and high-temperature) peridotites, as well as diamond-bearing dunites and harzburgites. The δ18O values of the metasomatized amphibole- and phlogopite-bearing peridotites also fall within the mantle interval. The fractionation of oxygen isotopes at a high temperatures is insignificant. Nevertheless, the equilibrated minerals of mantle peridotites demonstrate progressive depletion in δ18O in the orthopyroxene–clinopyroxene–garnet–olivine–spinel series in consistence with the crystal chemistry of these minerals. The δ18O of spinel varies from +3.8 to +4.8‰, while the oxygen isotope ratios of other minerals fall within the mantle interval. In particular, the δ18O of garnet is +5.3 ± 0.3‰.