Potassium isotopic evidence for a high-energy giant impact origin of the Moon
Potassium isotopic evidence for a high-energy giant impact origin of the Moon
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DOI：
10.1038/nature19341
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发表时间：
2016-10-27
期刊：
影响因子：
64.8
通讯作者：
Jacobsen, Stein B.
中科院分区：
文献类型：
--
作者：
Wang, Kun;Jacobsen, Stein B.
与其他行星体相比，地月系统具有独特的化学和同位素特征（1-3）;因此，任何成功的该系统起源模型都必须满足这些化学和同位素约束。与地球和太阳体成分相比，月球基本上缺乏钾等挥发性元素（4），长期以来一直被认为是灾难性的月球形成巨型撞击事件的结果（5）。挥发性元素耗尽的天体，如月球，在挥发性物质流失期间，预计会富含重钾同位素;然而，这种富集从未被发现（6）。在这里，我们报告了地球，月球和南极陨石的新的高精度钾同位素数据。我们发现，与地球和月岩相比，月球岩石中钾的重同位素明显富集（>2 sigma）（约为千分之0.4）。与地球和月岩相比，月岩中钾的重同位素富集，最好的解释是大量硅酸盐地球蒸汽在高于10巴的环境压力下不完全冷凝的结果。我们使用这些耦合的化学损失和同位素分馏的K的限制，比较两个最近的动态模型，用于解释相同的非质量依赖的同位素组成的地球和月球。我们的K同位素结果与低能盘平衡模型（7）不一致，但支持月球起源的高能、高角动量巨撞击模型（8）。高精度的钾同位素数据也可以作为“古气压计”，揭示月球形成事件的物理条件。
The Earth-Moon system has unique chemical and isotopic signatures compared with other planetary bodies(1-3); any successful model for the origin of this system therefore has to satisfy these chemical and isotopic constraints. The Moon is substantially depleted in volatile elements such as potassium compared with the Earth and the bulk solar composition(4), and it has long been thought to be the result of a catastrophic Moon-forming giant impact event(5). Volatile-element-depleted bodies such as the Moon were expected to be enriched in heavy potassium isotopes during the loss of volatiles; however such enrichment was never found(6). Here we report new high-precision potassium isotope data for the Earth, the Moon and chondritic meteorites. We found that the lunar rocks are significantly (>2 sigma) enriched in the heavy isotopes of potassium compared to the Earth and chondrites (by around 0.4 parts per thousand). The enrichment of the heavy isotope of potassium in lunar rocks compared with those of the Earth and chondrites can be best explained as the result of the incomplete condensation of a bulk silicate Earth vapour at an ambient pressure that is higher than 10 bar. We used these coupled constraints of the chemical loss and isotopic fractionation of K to compare two recent dynamic models that were used to explain the identical non-mass-dependent isotope composition of the Earth and the Moon. Our K isotope result is inconsistent with the low-energy disk equilibration model(7), but supports the high-energy, high-angular-momentum giant impact model(8) for the origin of the Moon. High-precision potassium isotope data can also be used as a 'palaeo-barometer' to reveal the physical conditions during the Moon-forming event.