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

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

★招待講演

*芳野 極¹、櫻井 萌²、坂本 直哉³、圦本 尚義⁴ (1.岡山大学地球物質科学研究センター、2.東京工業大学地球惑星科学科、3.北海道大学創成研究機構、4.北海道大学大学院理学研究院 自然史科学部門 地球惑星システム科学分野)

キーワード：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 FeH_x trapping the released hydrogen would become the main carrier of hydrogen into the deep mantle.