Japan Geoscience Union Meeting 2016

Presentation information

Print

International Session (Oral)

Symbol S (Solid Earth Sciences) » S-GC Geochemistry

[S-GC16] Volatile Cycles in the Deep Earth - from Subduction Zones to the Mantle and Core

Wed. May 25, 2016 1:45 PM - 3:15 PM 304 (3F)

Convener:*Hirochika Sumino(Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo), Takeshi Hanyu(Japan Agency for Marine-Earth Science and Technology, Department of Solid Earth Geochemistry), Yuji Sano(Division of Ocean and Earth Systems, Atmosphere and Ocean Research Institute, University of Tokyo), Colin Jackson(Geophysical Laboratory, Carnegie Institution of Washington), Chair:Jackson Colin(Carnegie Institution for Science Washington), Yuji Sano(Division of Ocean and Earth Systems, Atmosphere and Ocean Research Institute, University of Tokyo)

2:45 PM - 3:00 PM

[SGC16-05] Redox dehydration melting of mantle transition zone deduced from the H2O storage capacity

★Invited papers

*Takashi Yoshino1, Moe Sakurai2, Naoya Sakamoto3, Hisayoshi Yurimoto4 (1.Institute for Study of the Earth's Interior, Okayama University, 2.Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 3.Isotope Imaging Laboratory, Creative Research Institution, Hokkaido University, 4.Department of Natural History Sciences, Hokkaido University)

Keywords:redox condition, water storage capacity, dehydration

Knowledge of the H2O storage capacities of minerals forming mantle peridotite provides essential constraints on estimation of H2O content and the onset of hydrous partial melting in the mantle. In the mantle transition zone, wadsleyite can store significant amount of H2O in their crystal structures under extremely high oxygen fugacity. However, the H2O storage capacity has not been determined under the low oxygen fugacity predicted from the mantle transition zone7. Here we report that the H2O storage capacity of wadsleyite in equilibrium with the peridotite assemblage under lower oxygen fugacity is much smaller than that under higher one. Very low H2O storage capacity of wadsleyite can attribute to the low H2O activity in the melt. Considering the more reducing state in the deep mantle, dominant speciation of volatile phases is not H2O but H2. Low H2O activity in the reduced deep mantle requires that H2O storage capacity in the Earth’s mantle is much smaller than that predicted from the maximum H2O concentration determined under the high oxygen fugacity. The hydrated and oxidised subducted slab will induce “redox dehydration melting” through decrease of oxygen fugacity by the surrounding reduced mantle transition zone. H2O in the generated melt will be reduced to hydrogen through the oxidation of iron-bearing minerals. Fe-H melt or FeHx trapping the released hydrogen would become the main carrier of hydrogen into the deep mantle.