Existence of Water in the Earth's Upper Mantle and Generation of Ultrabasic Magma

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  • INOUE Toru
    Department of Earth Sciences, Faculty of Science, Ehime University

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  • 上部マントルにおける水の存在と超塩基性マグマの生成
  • ジョウブ マントル ニ オケル ミズ ノ ソンザイ ト チョウ エンキセイ マ

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Abstract

The possible existence of water in the upper mantle and its implications to the generation of ultrabasic (komatiite) magma have been discussed on the basis of the recent melting experiments in the systems pyrolite+H2O and Mg2SiO4 MgSiO3-H2O. The experiments demonstrated that presence of a certain amount of H2O changes the melting temperatures, liquidus phases, and melt compositions in these system. No hydrous phases were observed near the solidus temperature below 12GPa, whereas hydrous modified spinel was found near the solidus temperatures at 15.5GPa. The maximum content of H2O in this phase is estimated to be 3.3wt% from the microprobe analysis and crystallographic constraint. This suggests that a significant amount of H2O can be accommodated in the mantle transition zone. The melting phase relations suggest that komatiite magmas can be generated in the hydrous mantle at about 200km depth.

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