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Cooling of the Earth and core formation after the giant impact

机译：巨大撞击后地球的冷却和岩芯的形成

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摘要

Kelvin calculated the age of the Earth to be about 24 million years by assuming conductive cooling from being fully molten to its current state. Although simplistic, his result is interesting in the context of the dramatic cooling that took place after the putative Moon-forming giant impact, which contributed the final —10 per cent of the Earth's mass. The rate of accretion and core segregation on Earth as deduced from the U-Pb system is much slower than that obtained from Hf-W systematics, and implies substantial accretion after the Moon-forming impact, which occurred 45 ± 5Myr after the beginning of the Solar System. Here we propose an explanation for the two timescales. We suggest that the Hf-W timescale reflects the principal phase of core-formation before the giant impact. Crystallization of silicate perovskite in the lower mantle during this phase produced Fe~(3+), which was released during the giant impact, and this oxidation resulted in late segregation of sulphur-rich metal into which Pb dissolved readily, setting the younger U-Pb age of the Earth. Separation of the latter metal then occurred 30 ± 10 Myr after the Moon-forming impact. Over this time span, in surprising agreement with Kelvin's result, the Earth cooled by about 4,000 K in returning from a fully molten to a partially crystalline state.
机译：开尔文假设传导性冷却已完全融化到当前状态，因此计算出地球的年龄约为2400万年。尽管简单化，但在假定的形成月球的巨大撞击之后发生的戏剧性降温的背景下，他的结果很有趣，它最终贡献了地球质量的10％。从U-Pb系统推算出的地球上吸积和核心偏析的速度比从Hf-W系统获得的速度要慢得多，这意味着在月球形成撞击后发生了实质性的吸积，这是在月球开始45±5毫秒之后发生的。太阳系。在这里，我们对这两个时间尺度提出一个解释。我们建议，Hf-W时间尺度反映了巨大影响之前核心形成的主要阶段。在此阶段，在下地幔中硅酸盐钙钛矿的结晶产生了Fe〜（3+），并在巨大撞击中释放出来，这种氧化导致富硫金属的后期偏析，Pb容易溶解于其中，从而形成了较年轻的U-地球的Pb年龄。在月球撞击后30±10 Myr，后者发生分离。在这段时间里，与开尔文的结果令人惊讶地一致，地球从完全熔融状态返回到部分结晶状态，冷却了大约4,000K。

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  • 来源
    《Nature》 |2005年第7063期|p.1345-1348|共4页
  • 作者

    Bernard J. Wood; Alex N. Halliday;

  • 作者单位

    Department of Earth and Planetary Sciences, Macquarie University, New South Wales 2109, Australia;

  • 收录信息 美国《科学引文索引》(SCI);美国《工程索引》(EI);美国《生物学医学文摘》(MEDLINE);美国《化学文摘》(CA);
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类 自然科学总论;
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