Lunar tungsten isotopic evidence for the late veneer
Lunar tungsten isotopic evidence for the late veneer
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DOI：
10.1038/nature14360
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发表时间：
2015-04-23
期刊：
影响因子：
64.8
通讯作者：
Sprung, Peter
中科院分区：
文献类型：
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作者：
Kruijer, Thomas S.;Kleine, Thorsten;Sprung, Peter
根据最广为接受的月球起源理论，一次对地球的巨大撞击导致了月球的形成，同时也开启了地球核心形成的最后阶段（1）。核心的形成本应从地球原始地幔（即整体硅酸盐地球）中移除高度亲铁元素（HSE），然而HSE的丰度却高于预期（2）。对于这种过量的一种解释是，在核心形成之后，原始物质的“晚期薄层”被添加到了整体硅酸盐地球上（2）。为了检验这一假设，钨同位素因其两个原因而有用：首先，因为晚期薄层物质的W - 182 / W - 184比值与整体硅酸盐地球不同；其次，添加到地球上的物质相对于添加到月球上的比例更高（3）。因此，如果确实存在晚期薄层，那么整体硅酸盐地球和月球的W - 182 / W - 184比值必然不同。此外，形成月球的撞击也会造成W - 182的差异，因为撞击体具有不同W - 182 / W - 184比值的地幔和核心物质在巨大撞击过程中会与原地球混合。然而，月球的W - 182 / W - 184比值尚未被足够精确地测定，以识别晚期薄层或巨大撞击的特征。在此，我们利用更精确的测量技术表明，月球相对于现今的整体硅酸盐地球呈现出27±4ppm的W - 182过量。这种过量与从HSE系统分析推断出的具有一定总质量和成分的晚期薄层所导致的预期W - 182差异相符（2）。因此，我们的数据独立地表明，整体硅酸盐地球中的HSE丰度是在巨大撞击和核心形成之后确立的，正如晚期薄层假设所预测的那样。但是，出乎意料的是，我们发现在晚期薄层之前，整体硅酸盐地球和月球之间不存在W - 182异常，尽管巨大撞击本应导致这种异常出现。晚期薄层之前的整体硅酸盐地球和月球的均匀W - 182的起源是神秘的，对当前的月球起源模型构成了挑战。
According to the most widely accepted theory of lunar origin, a giant impact on the Earth led to the formation of the Moon, and also initiated the final stage of the formation of the Earth's core(1). Core formation should have removed the highly siderophile elements (HSE) from Earth's primitive mantle (that is, the bulk silicate Earth), yet HSE abundances are higher than expected(2). One explanation for this overabundance is that a 'late veneer' of primitive material was added to the bulk silicate Earth after the core formed(2). To test this hypothesis, tungsten isotopes are useful for two reasons: first, because the late veneer material had a different W-182/W-184 ratio to that of the bulk silicate Earth, and second, proportionally more material was added to the Earth than to the Moon(3). Thus, if a late veneer did occur, the bulk silicate Earth and the Moon must have different W-182/W-184 ratios. Moreover, the Moon-forming impact would also have created W-182 differences because the mantle and core material of the impactor with distinct W-182/W-184 would have mixed with the proto-Earth during the giant impact. However the W-182/W-184 of the Moon has not been determined precisely enough to identify signatures of a late veneer or the giant impact. Here, using more-precise measurement techniques, we show that the Moon exhibits a W-182 excess of 27 +/- 4 parts per million over the present-day bulk silicate Earth. This excess is consistent with the expected W-182 difference resulting from a late veneer with a total mass and composition inferred from HSE systematics(2). Thus, our data independently show that HSE abundances in the bulk silicate Earth were established after the giant impact and core formation, as predicted by the late veneer hypothesis. But, unexpectedly, we find that before the late veneer, no W-182 anomaly existed between the bulk silicate Earth and the Moon, even though one should have arisen through the giant impact. The origin of the homogeneous W-182 of the pre-late-veneer bulk silicate Earth and the Moon is enigmatic and constitutes a challenge to current models of lunar origin.