TEHORETICAL STUDY OF THE DIFFERENTIATION PROCESSES OF PROTOMANTLE AND EARLY EVOLUTION OF TERRESTRIAL PLANETS

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Achievement of this study is establishment of (1)a scenario for the early evolution based on cooling of a magma ocean, (2)criteria for the differentiation of the lower mantle. (1)A scenario for the early evolution : (a)Viscosity of partial melt changes drastically at the 'critical melt fraction'. Hence, magma oceans are divided into 2 categories, namely, the 'soft magma ocean' in which the melt fraction is higher than the critical melt fraction, and the 'hard magma ocean in which the melt fraction is lower than the critical melt fraction. (B)In a soft magma ocean, vigorous convection disturbs chemical differentiation. Without extensive heat source and/or atmospheric blanketing, this magma ocean cools into the hard magma ocean within 1 My. (c)In a hard magma ocean, less vigorous convection allows chemical differentiation. Owing to slow cooling, this stage continues more than 100-200My. Hence, the Earth's interior is in a hard magma ocean state just after the end of accretion. (d)At this stage, the surface of the Earth is covered by a water ocean and a very thin chilled crust. However, this crust is repeatedly destroyed by magmatic activity from the subsurface magma ocean and impacts of planetesimals. (2)Differentiation of the lower mantle : (a)Formation of a deep magma ocean does not necessary imply chemical differentiation at the lower mantle condition. (b)Specifically, without atmospheric blanketing, very rapid cooling suppresses chemical differentiation. (c)However, when the surface temperature is kept above 2200K by an atmospheric blanketing, chemical differentiation proceeds. Thus, differentiation of the lower mantle critically depends on the condition during accretion.