Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Strasse 10, D-48149 Münster, Germany.
Nature. 2015 Apr 23;520(7548):534-7. doi: 10.1038/nature14360. Epub 2015 Apr 8.
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. 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. One explanation for this overabundance is that a 'late veneer' of primitive material was added to the bulk silicate Earth after the core formed. To test this hypothesis, tungsten isotopes are useful for two reasons: first, because the late veneer material had a different (182)W/(184)W ratio to that of the bulk silicate Earth, and second, proportionally more material was added to the Earth than to the Moon. Thus, if a late veneer did occur, the bulk silicate Earth and the Moon must have different (182)W/(184)W ratios. Moreover, the Moon-forming impact would also have created (182)W differences because the mantle and core material of the impactor with distinct (182)W/(184)W would have mixed with the proto-Earth during the giant impact. However the (182)W/(184)W 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 (182)W excess of 27 ± 4 parts per million over the present-day bulk silicate Earth. This excess is consistent with the expected (182)W difference resulting from a late veneer with a total mass and composition inferred from HSE systematics. 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 (182)W 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 (182)W of the pre-late-veneer bulk silicate Earth and the Moon is enigmatic and constitutes a challenge to current models of lunar origin.
根据最广为接受的月球起源理论，地球曾遭受一次巨大撞击，由此形成了月球，并开启了地球核心形成的最后阶段。核心形成过程本应将亲铁元素（HSE）从地球原始地幔（即整个硅酸盐地幔）中移除，但实际上 HSE 的丰度却高于预期。对于这种丰度过剩的一种解释是，在核心形成之后，有一层原始物质的“晚期覆盖层”被添加到整个硅酸盐地幔中。为了验证这一假设，钨同位素有两个原因是有用的：首先，因为晚期覆盖层物质的（182）W/（184）W 比值与整个硅酸盐地幔不同；其次，与月球相比，更多的物质被添加到地球上。因此，如果确实存在晚期覆盖层，那么整个硅酸盐地幔和月球的（182）W/（184）W 比值必须不同。此外，月球形成的撞击也会产生（182）W 差异，因为撞击体的地幔和核心物质的（182）W/（184）W 比值不同，在与原始地球发生大型撞击时会混合在一起。然而，月球的（182）W/（184）W 比值尚未精确确定，无法确定晚期覆盖层或大型撞击的特征。在这里，我们使用更精确的测量技术，表明月球的（182）W 相对于现今整个硅酸盐地幔存在 27±4 百万分比的过剩。这一过剩与通过 HSE 系统学推断的晚期覆盖层的总质量和组成所预期的（182）W 差异一致。因此，我们的数据独立表明，HSE 在整个硅酸盐地幔中的丰度是在大型撞击和核心形成之后建立的，这与晚期覆盖层假说的预测一致。但是，出乎意料的是，我们发现，在晚期覆盖层之前，整个硅酸盐地幔和月球之间不存在（182）W 异常，尽管这种异常本应通过大型撞击而产生。在晚期覆盖层之前，整个硅酸盐地幔和月球之间（182）W 的均匀性起源是神秘的，这对月球起源的现有模型构成了挑战。
