[S-IT16] Structure and Dynamics of Earth and Planetary Mantles

[SIT16-01] Rapid change in the surface environment during the Hadean driven by the chemically heterogeneous mantle

*宮崎 慶統¹、是永 淳¹ (1.イェール大学)

キーワード：冥王代、地球ダイナミクス、マグマオーシャンの分化

The Hadean eon is considered to be a dynamic period, where the surface environment evolved from being uninhabitable to habitable within <500 Myr. Immediately after the solidification of a magma ocean, a thick atmosphere of greenhouse gases likely maintained a fiendish surface condition, but by the beginning of Archean, Earth is expected to have water oceans with a surface temperature similar to the present day. The rapid sequestration of carbon dioxide has been considered as a key to this evolution, but how >100 bar of carbon dioxide, released during the solidification of a magma ocean, was removed by the end of Hadean remains uncertain. We address this question by delineating the mode of Hadean geodynamics. The sequestration of carbon is governed by the subduction of carbonates formed at ocean basins, and its efficiency depends on plate velocity. The viscosity structure of the mantle plays a key role in determining geodynamics, so we first consider how the degree of mantle hydration changed during the solidification of a magma ocean by building a new kind of degassing model. Our results suggest that a mantle was not only hotter but also wetter than the present-day mantle. Also, the mantle was likely dominated by high-magnesium pyroxenites as a result of the fractional solidification of a magma ocean, and its high melting temperature likely resulted in a thinner depleted lithospheric mantle than a pyrolitic mantle. With a wet mantle dominated by high-magnesium pyroxenites, plate velocity exceeds 50 cm/yr, and Earth likely produced a habitable environment within ~100 Myr. With either a dry or a pyrolitic mantle, however, the sequestration of >100 bar of carbon dioxide may have taken longer than ~1.5 Gyr. Such a chemically heterogeneous mantle would also produce oceanic crust rich in olivine, which is reactive with ocean water and would promote serpentinization. This resembles conditions in the Lost City Hydrothermal Field, and thus environment suitable for the emergence of early life likely existed globally in the Hadean.