Patterns in the hydrogen and trace element compositions of mantle olivines |
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Authors: | M Kurosawa H Yurimoto S Sueno |
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Institution: | (1) Institute of Geoscience, The University of Tsukuba, Tsukuba, Ibaraki 305, Japan, JP |
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Abstract: | The concentrations of hydrogen and the other trace elements in olivines from mantle xenoliths have been determined by secondary
ion mass spectrometry (SIMS) for clarifying the incorporation mechanism and the behavior 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 spectra of the olivine
crystals show no effects of the weathering or secondary alteration. The hydrogen is distributed homogeneously among olivine
grains in each mantle xenolith. However, the hydrogen contents of 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 hydrogen content of xenolithic olivines does not attain equilibrium with water in the host magma during
the transportation from the Earth's mantle to the surface, and is taken as a reflection of the hydrogen condition in the mantle.
Correlations of hydrogen with trivalent cation contents in garnet peridotitic olivines indicate the incorporation of hydrogen
into mantle olivines by a coupled substitution mechanism, with the hydrogen present in the form of hydroxyl in oxygen positions
adjacent to the M site vacancies. The hydrogen content of xenolithic olivines increases with pressure but decreases with increasing
temperature, suggesting importance of olivine as a water reservoir at low temperature regions such as in subducting slabs.
Received August 15, 1995/Revised, accepted November 19, 1996 |
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