The Response of the Earth to the Terminal Cataclysm

地球是通过一系列较小行星的撞击而成长起来的。上一次重大事件是一个火星大小的物体撞击了约90%完整的地球。这次撞击涉及足够的能量，将足够的物质发射到轨道上，形成月球并熔化剩余的地球。如果不是之前，这次全球融化事件允许地球中多余的熔体下沉到中心并形成核心。这完成了主要的建设阶段，但大约6亿年后，一系列较小的，但不是微不足道的影响击中了地球。这被称为“终端灾难”，最初是在39亿年前从月球上的陨石坑年龄集群中识别出来的。虽然物质上不那么有形，但对地球的影响是深远的，至少从资源的角度来看是如此。贵金属如金和铂相对于硅酸盐熔体或晶体相（硅酸盐矿物构成地球的外层）具有极高的存在于金属熔体中的偏好。金属熔化物下沉形成地核，因此携带着地球最初的大部分Au和Pt。在地核形成锁定了最初的储藏之后，人类目前可以获得的Au和Pt随着终极大灾变而来。由于最终灾难没有引发进一步的地核形成，这种所谓的晚期单板仍然存在于地球的外部硅酸盐部分。晚期单板中的贵金属现在很好地分布在整个地球上，这个项目将遵循终端灾难的重新分配。在这样做的过程中，我们将了解地幔（对流，地球的硅酸盐部分）在地球历史的前半部分的行为方式。我们还认为，终极大灾变可能引发了我们目前所看到的板块构造的现代对流形式的开始。我们将调查这个想法，看看其他古代化学签名是否与晚期单板同时均匀化。一个积极的结果将表明这两个过程之间的联系，并暗示终端大灾变在启动这一过程。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（发现于老式灯泡和切割工具中）和Mo（用作工业润滑剂）。晚期陨石物质的同位素比值与末次灾变前地球的同位素比值不同。差异很小，但我们的新技术具有适当的精度来检测这些差异。使用同位素比值来追踪这一过程的优势在于，它们基本上只通过混合而发生变化，而很少受到其他地质过程的影响。通过研究一系列越来越年轻的岩石中Mo和W的同位素比值，我们希望追踪晚期单板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.