Hydrous Volatiles in Upper Mantle Minerals

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The concentration and chemical speciation of volatile components in the Earth's upper mantle play an important role in geologic processes. Despite their importance, we know little about their actual modes of storage in the various geochemical reservoirs. Previously, infrared spectroscopy was used to establise that variable quantities of OG occur in pyroxenes, garnet and olivine brought to the Earth's surface as xenoliths in alkalic magmas. Preliminary indications are that the OH concentration of these minerals is correlated with their environmmet of formation and may thus reflect ambient hydrous volatioe activity. The PIs plan to determine the actual OH content of important mantle minerals, concentrating on the subcontinental mantle of southern Africa. They will calibrate the IR method using a variety of methods. This will allow them to quantify in absolute terms the abundance of inter-mineral partitioning of OH in these rocks and perhaps to provide D/H ratios. In order to relate the observed OH contents of the samples to volatile activity of the sources, they will measure the OH contents of garnets grown synthetically under controlled conditions of pressure, temperature and volatile activity in a piston cylinder apparatus. The results are expected to have an important bearing on understanding how volatiles are distributed in the upper mantle as well as how they behave int eh processes of melt generation and subduction.