INCORPORATION OF HYDROXYL IN UPPER-MANTLE CLINOPYROXENES

WATER (and hydroxyl, OH) plays an important part in determining the properties of minerals and melts in the Earth's upper mantle. The main hydroxyl-bearing phases found in rocks from the upper mantle, phlogopite and amphibole, are not believed to exist in significant quantities at depth. Some of the less abundant phases found in these rocks contain small amounts of hydrous components 1-3, but do not constitute an important reservoir for water. Traces of hydroxyl have been found in common mantle phases 4-6, but not at concentrations high enough to account for the amount of water thought to be present at depth. An exception is suggested by a report 7 that the pyroxene (omphacite) in an eclogite nodule from the Roberts Victor kimberlite pipe contains up to 1,000 p.p.m. OH. Here we show that some of this hydroxyl is associated with cation vacancies in sodic clinopyroxene, and that these pyroxenes may be an important reservoir for hydrous components in the upper mantle.