FROM SNOWBALL TO PHANEOROZIC EARTH

The Late Proterozoic Snowball Earth disappeared at the beginning of Phanerozoic time; such dramatic change may have resulted from the return flow of seawater into the mantle beginning at 750 Ma, as a consequence of the cooling of the Earth and start of the main glacial epoch. Phase relations for minerals in the system MORB + H2O and peridotites + H2O suggest that old, cold subducting oceanic slabs would transport surface water through ultrahigh-P (UHP) hydrous silicates to the mantle transition zone at depths of 410-660 km. This zone could store abundant H2O, inasmuch as hydrous ? and ? phases of the olivine composition may contain up to 2 to 3 wt% water. Geotherms computed for rocks of ancient subduction zones from regional metamorphic belts around the world record such a warm-to-cold transition in and below the lithosphere at 750 Ma. The run-away cooling of the Earth's interior along consuming plate boundaries started at 750 Ma by propagation of a hydration front into the mantle from 70 km depth to 410-660 km through a narrow channel above the Wadati-Benioff plane, or directly by the descent of hydrated slab peridotites. Resultant hydration of the mantle wedge and adjacent regions, caused a rapid sea-level drop during the interval of 750-600 Ma, and large landmasses appeared. This change was due to the increase of water content from 0.5 for amphibole peridotite to 6.5 wt% for antigorite peridotite in the wedge mantle, and to the pop-corn effect to push-up continental margins. The total seawater lost since then is estimated to have lowered the sea-level about 600 m. Introduction of seawater into the mantle drastically lowered the melting temperature and viscosity of mantle materials, both of which reactivated the plate and plume tectonic processes. Surface volcanism released mantle CO2, causing the greenhouse effect to melt global glaciers of the snowball Earth since about 750 Ma. The emergence of huge continental landmasses diversified surface environments; erosion and deposition of sediments became significant as in the present-day Earth. Extensive formation of sedimentary basins and accretionary complexes along consuming plate boundaries drastically increased free oxygen in the atmosphere by the burial organic matters in those sediments, which was critical to the evolution of large multicellar organisms.