Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA.
Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis, Missouri 63130, USA.
Nature. 2016 Oct 27;538(7626):487-490. doi: 10.1038/nature19341. Epub 2016 Sep 12.
The Earth-Moon system has unique chemical and isotopic signatures compared with other planetary bodies; 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, and it has long been thought to be the result of a catastrophic Moon-forming giant impact event. 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. 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 (>2σ) 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, but supports the high-energy, high-angular-momentum giant impact model 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.
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；因此，任何成功的地月系统起源模型都必须满足这些化学和同位素约束。月球与地球和太阳的整体成分相比，挥发性元素（如钾）明显匮乏，长期以来一直被认为是月球形成的灾难性巨撞击事件的结果。像月球这样的挥发性元素匮乏的天体，在挥发性元素损失过程中，预计会富含重钾同位素；然而，这种富集会从未被发现。在这里，我们报告了地球、月球和球粒陨石的新的高精度钾同位素数据。我们发现，与地球和球粒陨石相比，月球岩石中钾的重同位素明显（>2σ）富集（约 0.4 个千分位）。与地球和球粒陨石相比，月球岩石中钾的重同位素的富集，可以最好地解释为在环境压力高于 10 巴的情况下，原始地核蒸气不完全凝结的结果。我们利用这些化学损失和 K 同位素分馏的耦合约束，比较了两种最近用于解释地球和月球相同的非质量依赖型同位素组成的动力学模型。我们的 K 同位素结果与低能量盘平衡模型不一致，但支持了用于解释月球起源的高能、高角动量巨撞击模型。高精度钾同位素数据也可用作“古压力计”，以揭示月球形成过程中的物理条件。
