PATTERNS IN THE HYDROGEN AND TRACE-ELEMENT COMPOSITIONS OF MANTLE OLIVINES

The concentrations of hydrogen and the other trace elements in olivine s from mantle xenoliths have been determined by secondary ion mass spe ctrometry (SIMS) for clarifying the incorporation mechanism and the be havior of the hydrogen. The hydrogen contents in olivines from mantle xenoliths range from 10 to 60 ppm wt. H2O and the concentration range is consistent with the previous infrared (IR) spectroscopic data. IR s pectra of the olivine crystals show no effects of the weathering or se condary alteration. The hydrogen is distributed homogeneously among ol ivine grains in each mantle xenolith. However, the hydrogen contents o f the olivine crystals are less than those for the olivine phenocrysts crystallized from the host magma. Olivine inclusions in diamonds also show similar hydrogen contents to the xenolithic olivines. Thus the h ydrogen content of xenolithic olivines does not attain equilibrium wit h water in the host magma during the transportation from the Earth's m antle to the surface, and is taken as a reflection of the hydrogen con dition in the mantle. Correlations of hydrogen with trivalent cation c ontents in garnet peridotitic olivines indicate the incorporation of h ydrogen into mantle olivines by a coupled substitution mechanism, with the hydrogen present in the form of hydroxyl in oxygen positions adja cent to the M site vacancies. The hydrogen content of xenolithic olivi nes increases with pressure but decreases with increasing temperature, suggesting importance of olivine as a water reservoir at low temperat ure regions such as in subducting slabs.