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The tungsten isotopic composition of the Earth's mantle before the terminal bombardment

机译：末期轰击前地球地幔的钨同位素组成

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Many precious, 'iron-loving' metals, such as gold, are surprisingly abundant in the accessible parts of the Earth, given the efficiency with which core formation should have removed them to the planet's deep interior~1. One explanation of their over-abundance is a 'late veneer'—a flux of meteorites added to the Earth after core formation as a 'terminal' bombardment that culminated in the cratering of the Moon~2. Some 3.8 billion-year-old rocks from Isua, Greenland, are derived from sources that retain an isotopic memory of events pre-dating this cataclysmic meteorite shower~(3'4). These Isua samples thus provide a window on the composition of the Earth before such a late veneer and allow a direct test of its importance in modifying the composition of the planet. Using high-precision (less than 6 parts per million, 2 standard deviations) tungsten isotope analyses of these rocks, here we show that they have a isotopic tungsten ratio ~(182)W~(184)W that is significantly higher (about 13 parts per million) than modern terrestrial samples. This finding is in good agreement with the expected influence of a late veneer. We also show that alternative interpretations, such as partial remixing of a deep-mantle reservoir formed in the Hadean eon~(5'6) (more than four billion years ago) or core-mantle interaction~7, do not explain the W isotope data well. The decrease in mantle ~(182)W~(184)W occurs during the Archean eon (about four to three billion years ago), potentially on the same timescale as a notable decrease in ~(142)Nd~(144)Nd (refs 3 and 6). We speculate that both observations can be explained if late meteorite bombardment triggered the onset of the current style of mantle convection.
机译：考虑到核心形成的效率，应该将许多珍贵的“喜欢铁”的金属（例如金）大量存在于地球的可及部分，从而将它们移至地球的深层内部〜1。导致它们过剩的一种解释是“晚期单板”，即形成核心之后，作为“终末”轰击而在地球上形成陨石通量的陨石，最终导致月球2号陨石坑。来自格陵兰岛伊苏阿的大约38亿年前的岩石，其来源保留了在此大灾难性陨石阵雨前的同位素记录[3'4]。因此，这些Isua样品在这样的晚期单板之前提供了一个有关地球组成的窗口，并可以直接测试其在改变行星组成方面的重要性。使用这些岩石的高精度（小于百万分之六的百万分之2的标准差）钨同位素分析，这里我们显示它们的同位素钨比〜（182）W〜（184）W显着更高（约13百万分之几）。这一发现与后期饰面板的预期影响非常吻合。我们还表明，替代解释（例如，在哈德系eon〜（5'6）（超过40亿年前）中形成的深地幔储层的部分再混合或岩心-地幔相互作用〜7）不能解释W同位素数据很好。地幔〜（182）W〜（184）W的减少发生在太古宙时代（大约四到三十亿年前），可能与〜（142）Nd〜（144）Nd的显着减少的时间范围相同（参考3和6）。我们推测，如果晚陨石轰炸触发了当前地幔对流形式的开始，那么两种解释都可以解释。

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  • 来源
    《Nature》 |2011年第7363期|p.195-198|共4页
  • 作者

    Matthias Willbold; Tim Elliott; Stephen Moorbath;

  • 作者单位

    Bristol Isotope Group, School of Earth Sciences, Wills Memorial Building, Queens Road, University of Bristol, Bristol BS8 1RJ, UK;

    Bristol Isotope Group, School of Earth Sciences, Wills Memorial Building, Queens Road, University of Bristol, Bristol BS8 1RJ, UK;

    Department of Earth Sciences, South Parks Road, University of Oxford,Oxford 0X1 3AN.UK;

  • 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);
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