Touboul M, Kleine T, Bourdon B, Palme H, Wieler R
Institute for Isotope Geochemistry and Mineral Resources, Department of Earth Sciences, Eidgenössische Technische Hochschule Zurich, Clausiusstrasse 25, 8092 Zurich, Switzerland.
Nature. 2007 Dec 20;450(7173):1206-9. doi: 10.1038/nature06428.
The Moon is thought to have formed from debris ejected by a giant impact with the early 'proto'-Earth and, as a result of the high energies involved, the Moon would have melted to form a magma ocean. The timescales for formation and solidification of the Moon can be quantified by using 182Hf-182W and 146Sm-142Nd chronometry, but these methods have yielded contradicting results. In earlier studies, 182W anomalies in lunar rocks were attributed to decay of 182Hf within the lunar mantle and were used to infer that the Moon solidified within the first approximately 60 million years of the Solar System. However, the dominant 182W component in most lunar rocks reflects cosmogenic production mainly by neutron capture of 181Ta during cosmic-ray exposure of the lunar surface, compromising a reliable interpretation in terms of 182Hf-182W chronometry. Here we present tungsten isotope data for lunar metals that do not contain any measurable Ta-derived 182W. All metals have identical 182W/184W ratios, indicating that the lunar magma ocean did not crystallize within the first approximately 60 Myr of the Solar System, which is no longer inconsistent with Sm-Nd chronometry. Our new data reveal that the lunar and terrestrial mantles have identical 182W/184W. This, in conjunction with 147Sm-143Nd ages for the oldest lunar rocks, constrains the age of the Moon and Earth to Myr after formation of the Solar System. The identical 182W/184W ratios of the lunar and terrestrial mantles require either that the Moon is derived mainly from terrestrial material or that tungsten isotopes in the Moon and Earth's mantle equilibrated in the aftermath of the giant impact, as has been proposed to account for identical oxygen isotope compositions of the Earth and Moon.
月球被认为是由早期“原始”地球遭受巨大撞击后喷出的碎片形成的，由于撞击涉及的能量极高，月球会熔化形成岩浆海洋。月球形成和固化的时间尺度可以通过使用182铪 - 182钨和146钐 - 142钕计时法来量化，但这些方法得出了相互矛盾的结果。在早期研究中，月球岩石中的182钨异常归因于月球地幔中182铪的衰变，并被用于推断月球在太阳系形成后的大约头6000万年之内固化。然而，大多数月球岩石中占主导地位的182钨成分主要反映了月球表面在宇宙射线照射期间通过181钽的中子俘获产生的宇宙成因，这使得基于182铪 - 182钨计时法的可靠解释变得复杂。在这里，我们展示了不含任何可测量的源自钽的182钨的月球金属的钨同位素数据。所有金属的182钨/184钨比值相同，表明月球岩浆海洋在太阳系形成后的大约头6000万年之内没有结晶，这与钐 - 钕计时法不再矛盾。我们的新数据表明，月球和地球地幔具有相同的182钨/184钨。这与最古老月球岩石的147钐 - 143钕年龄相结合，将月球和地球的年龄限制在太阳系形成后的数百万年。月球和地球地幔相同的182钨/184钨比值要求要么月球主要源自地球物质，要么月球和地球地幔中的钨同位素在巨大撞击之后达到平衡，就像为解释地球和月球相同的氧同位素组成而提出的那样。
