The Response of the Earth to the Terminal Cataclysm

地球是通过一系列小行星的撞击而形成的。上一次重大事件是火星大小的物体撞击了90%的地球。这次撞击产生了足够的能量，将足够的物质发射到轨道上，形成月球，并融化剩下的地球。如果不是在此之前，这次全球融化事件使地球上多余的融化物下沉到中心形成地核。这完成了主要的建造阶段，但大约6亿年之后，一系列较小但并非微不足道的撞击袭击了地球。这被称为“终极大灾难”，最初是通过月球上的陨石坑群发现的，当时距离现在还有39亿年。虽然不那么有形，但至少从资源的角度来看，对地球的影响是深远的。相对于硅酸盐熔体或晶体相（硅酸盐矿物构成地球的外层），黄金和铂等贵金属更倾向于存在于金属熔体中。金属熔化后下沉形成了地核，因此带走了地球上绝大多数最初的金和铂储量。目前人类可以获得的金和铂是在地核形成后锁定了最初的宝藏之后，随着最终的大灾难而到来的。由于末梢大灾变并没有引发地核的进一步形成，这个所谓的晚期饰面仍然在地球的外层硅酸盐部分。来自后期贴面的贵金属现在很好地分布在整个地球上，这个项目将遵循这个重新分配从终端大灾难。通过这样做，我们将了解地幔（地球的对流硅酸盐部分）在地球历史上半期的行为方式。我们还认为，最终大灾难可能引发了现代形式的对流，我们现在看到的是板块构造。我们将通过观察其他古代化学特征是否与晚饰面同时均匀化来调查这一想法。一个积极的结果将表明这两个过程之间存在联系，并暗示这一过程的开始是“终极大灾难”。这项工作将通过高精度测量一些所谓的亲铁元素的同位素比率来完成，即W（存在于老式灯泡和切割工具中）和Mo（用作工业润滑剂）。在末世大灾变前，晚期贴面所传递的陨石物质的同位素比值与地球的同位素比值不同。差异很小，但我们的新技术有适当的精度来检测这些差异。使用同位素比率来追踪这一过程的优势在于，它们基本上只在混合过程中发生变化，而其他地质过程对它们的影响很小。通过在一系列越来越年轻的岩石中检查Mo和W的同位素比率，我们希望追踪晚期饰面与地幔其余部分的混合。

The Earth grew via a series of impacts of smaller planets. The last major event saw a Mars-sized object hit the ~90% complete Earth. This impact involved sufficient energy to launch enough material into orbit to form the moon and melt the remaining Earth. If not before, this global melting event allowed the excess melt in the Earth to sink to the centre and form the core. This finished the main stage of construction but some 600 million years afterwards, a series of smaller, but not insubstantial impacts hit the Earth. This has been dubbed the Terminal Cataclysm and was originally recognised from the clustering of cratering ages on the moon at 3.9 billion years before present. Although less physically tangible, the influence on the Earth has been profound, at least from a resources perspective. Precious metals such as gold and platinum have an extremely high preference for residing in a metallic melt relative to silicate melt or crystal phases (silicate minerals comprise the outer layers of the Earth). Metallic melts than sank to produce the core, thus carried with them the vast majority of the Earth's initial inventory of Au and Pt. The Au and Pt that is currently accessible to humanity arrived with the Terminal Cataclysm, after core formation had locked away the original hoard. Since the Terminal Cataclym did not trigger further core formation, this so-called Late Veneer remained within the outer, silicate portion of the Earth. The precious metals from the late veneer are now well distributed throughout the Earth and this project will follow this redistribution from the Terminal Cataclysm. In doing so we will learn about the way in which the mantle (convecting, silicate portion of the Earth) behaves in the first half of Earth History. We also think that the Terminal Cataclysm may have triggered the onset of the modern form of convection we currently see as plate tectonics. We will investigate this idea by seeing if other ancient chemical signatures were homogenised at the same time as the Late Veneer. A positive result would suggest a link between the two processes and implicate the Terminal Cataclysm in starting this process.The method by which this work will be done is by high precision measurements of the isotope ratios of some of the so-called siderophile elements, namely W (as founded in old-style light bulbs and cutting tools) and Mo (used as an industrial lubricant). The isotope ratios of the meteorite material delivered by in the late veneer has different isotope ratios to the Earth before the Terminal Cataclysm. The differences are small but our new techniques have suitable precision to detect these. The strength of using isotope ratios to trace this process is that they are essentially changed only by mixing and little by other geological processes. By examining the isotope ratios of Mo and W in a sequence of increasingly young rocks, we hope to trace the mixing of the late veneer with the rest of the mantle.