The tungsten isotopic composition of the Earth’s mantle before the terminal bombardment
The tungsten isotopic composition of the Earth’s mantle before the terminal bombardment
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DOI：
10.1038/nature10399
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发表时间：
2011-09
期刊：
影响因子：
64.8
通讯作者：
M. Willbold;T. Elliott;S. Moorbath
中科院分区：
文献类型：
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作者：
M. Willbold;T. Elliott;S. Moorbath
考虑到地核形成的效率，许多珍贵的“亲铁”金属，如黄金，在地球可到达的地方储量惊人地丰富，这些金属本应被带到地球的深处。对它们数量过多的一种解释是“后期贴面”——在地核形成后，作为“终端”轰击的陨石流入地球，最终导致月球陨石坑的形成。来自格陵兰岛伊苏亚的一些38亿年前的岩石，其来源保留了这次灾难性陨石阵雨之前事件的同位素记忆。因此，这些Isua样本提供了一个窗口，了解地球在如此晚的贴面之前的组成，并允许对其在修改地球组成方面的重要性进行直接测试。通过对这些岩石进行高精度（小于百万分之6,2个标准差）的钨同位素分析，我们发现它们的同位素钨比182w /184W明显高于现代陆地样品（约为百万分之13）。这一发现与预期的晚贴面影响是一致的。我们还表明，其他解释，如冥古宙形成的深部地幔储层的部分再混合（超过40亿年前）或核-地幔相互作用，都不能很好地解释W同位素数据。地幔182w /184W的减少发生在太古宙（约40至30亿年前），可能与142 /144Nd的显著减少在同一时间尺度上（参考文献和）。我们推测，如果晚期陨石轰击触发了当前地幔对流的开始，这两个观测结果都可以解释。
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. 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. 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,. 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 ratio182W/184W 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,(more than four billion years ago) or core–mantle interaction, do not explain the W isotope data well. The decrease in mantle182W/184W occurs during the Archean eon (about four to three billion years ago), potentially on the same timescale as a notable decrease in142Nd/144Nd (refs and ). We speculate that both observations can be explained if late meteorite bombardment triggered the onset of the current style of mantle convection.