Rapid change in surface environment during the Hadean is facilitated by the exposure of high-magnesium peridotites

Abstract:

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 whether >100 bar of carbon dioxide, released during the solidification of a magma ocean, was removed within 500 Myr 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 peridotites as a result of the fractional solidification of a magma ocean, and its high melting temperature likely resulted in the lack of dry and stiff depleted lithospheric mantle. With a wet mantle dominated by high-magnesium peridotites, plate velocity exceeds 200 cm/yr, and Earth likely produced a habitable environment within 500 Myr. With either a dry or a pyrolitic mantle, however, the sequestration of >100 bar of carbon dioxide may have taken longer than the geological timescale. This also has an implication for early life. Because a mantle with high-Mg# does not melt during its upwelling, peridotites may have been extensively exposed to the surface, which reacts with ocean water to 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.