Hydrogen Contents of Mantle Minerals from Peridotite Xenoliths of Ichinomegata Volcano, NE Japan

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Hydrogen contents of olivine, clinopyroxene, and orthopyroxene from peridotite and pyroxenite xenoliths from calc-alkaline andesite of Ichinomegata volcano have been measured by using FTIR technique. Positions of bands in the infrared spectra resemble those observed previously for the studied minerals. Olivine contains <17 ppm wt. H_2O, whereas clinopyroxene and orthopyroxene contain significant amount of H_2O (359-647 and 184-277 ppm wt., respectively). Partition coefficient of H_2O between clinopyroxene and orthopyroxene was found to vary between 1.8 and 3.2. We did not observe systematic compositional (major elements in bulk rocks and minerals) or textural dependences of hydrogen contents in minerals. The low hydrogen contents of olivine are consistent with hydrogen loss during transportation of xenoliths to the surface. The high hydrogen contents of pyroxenes are consistent with strong metasomatic enrichment of the mantle wedge beneath the Japan arc. However, hydration of nominally anhydrous pyroxenes may reflect earlier events than hydration of mantle wedge by metasomatic agents producing modal amphibole in peridotite and abundant uppermost mantle/lower crust amphibolites. Pyroxenes from Ichinomegata contain nearly maximum amount of H_2O among the others from mantle xenoliths in alkaline basalts and kimberlites.論文Article
東北大学の論文