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

这项研究的成果是建立了(1)基于岩浆海洋冷却的早期演化假说，(2)下地幔的分异标准。(1)早期演化的一种情景：(A)部分熔体的粘度在“临界熔体分数”处发生剧烈变化。因此，岩浆海洋可分为两类，即熔体分数高于临界熔体分数的“软岩浆海洋”和熔体分数低于临界熔体分数的“硬岩浆海洋”。(B)在软岩浆海洋中，强烈的对流干扰了化学分异。在没有广泛的热源和/或大气覆盖的情况下，这些岩浆海洋在1英里内冷却成坚硬的岩浆海洋。(C)在坚硬的岩浆海洋中，不那么强烈的对流允许化学差异。由于降温缓慢，这一阶段持续了100-200多米。因此，地球内部在吸积作用刚刚结束后就处于坚硬的岩浆海洋状态。(D)在这个阶段，地球表面覆盖着海水海洋和非常薄的冰壳。然而，这个地壳被来自地下岩浆海洋的岩浆活动和行星体的撞击反复破坏。(2)下地幔的分异作用：(A)深部岩浆海洋的形成不一定意味着下地幔条件下的化学分异。(B)具体地说，在没有大气覆盖的情况下，非常迅速的冷却抑制了化学分化。(C)然而，当表面温度通过大气覆盖保持在2200K以上时，化学分化就会进行。因此，下地幔的分异很大程度上取决于增生过程中的条件。

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.