Core formation in planetesimals triggered by permeable flow
Core formation in planetesimals triggered by permeable flow
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DOI：
10.1038/nature01459
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发表时间：
2003-03
期刊：
影响因子：
64.8
通讯作者：
T. Yoshino;M. Walter;T. Katsura
中科院分区：
文献类型：
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作者：
T. Yoshino;M. Walter;T. Katsura
陨石的钨同位素组成表明，星子的核心形成发生在太阳系形成的几百万年之内。但岩心的形成需要一种分离金属的机制，而在富含橄榄石的基体中，熔融铁合金的“润湿”特性被认为可以阻止可渗透流体的分离，除非硅酸盐本身部分熔融。然而，超过渗透阈值的过量液态金属可能会在固体基体中产生渗透性，从而导致偏析。在这里，我们报告了约5 vol的熔融铁硫化合物的渗透阈值。根据在高压和高温下进行的电导率测量。我们得出的结论是，通过短寿命放射性核素的衰变在星子内加热可以使温度升高到铁硫熔点（~ 1000℃）以上，从而在钨同位素指示的短时间内通过可渗透流动触发铁合金的偏析。我们推断，半径大于30公里的星子和更大的行星胚胎很早就形成了核心，这些物体可能包含了原行星星云陆地区域的大部分质量。因此，地球和其他类地行星很可能是由先前分化的星子吸积而形成的，相应地，地球的核心可能被视为预先形成的核心的混合复合物。
The tungsten isotope composition of meteorites indicates that core formation in planetesimals occurred within a few million years of Solar System formation,. But core formation requires a mechanism for segregating metal, and the ‘wetting’ properties of molten iron alloy in an olivine-rich matrix is thought to preclude segregation by permeable flow unless the silicate itself is partially molten,,. Excess liquid metal over a percolation threshold, however, can potentially create permeability in a solid matrix, thereby permitting segregation. Here we report the percolation threshold for molten iron–sulphur compounds of approximately 5 vol.% in solid olivine, based on electrical conductivity measurements madein situat high pressure and temperature. We conclude that heating within planetesimals by decay of short-lived radionuclides can increase temperature sufficiently above the iron–sulphur melting point (∼1,000 °C) to trigger segregation of iron alloy by permeable flow within the short timeframe indicated by tungsten isotopes. We infer that planetesimals with radii greater than about 30 km and larger planetary embryos are expected to have formed cores very early, and these objects would have contained much of the mass in the terrestrial region of the protoplanetary nebula. The Earth and other terrestrial planets are likely therefore to have formed by accretion of previously differentiated planetesimals, and Earth's core may accordingly be viewed as a blended composite of pre-formed cores.