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。在上一次重大事件中，一个火星大小的物体撞击了大约90%的地球。这次撞击涉及足够的能量，足以将足够的物质发射到轨道上，形成月球并融化剩余的地球。如果不是在此之前，这一全球融化事件使得地球上多余的融化沉入中心，形成了核心。这完成了建造的主要阶段，但大约6亿年后，一系列规模较小但并不是无关紧要的撞击撞击了地球。这被称为末日大灾变，最初是从距今39亿年前月球上的陨石坑年龄集群中识别出来的。尽管物质上不那么有形，但至少从资源的角度来看，对地球的影响是深远的。与硅酸盐熔体或晶相(硅酸盐矿物构成地球外层)相比，黄金和铂金等贵金属更倾向于存在于金属熔体中。金属熔体然后下沉产生地核，因此携带了地球上最初库存的绝大多数金和铂。在核心形成锁定了原始的储藏之后，目前人类可以接触到的Au和Pt伴随着末日大灾变来到这里。由于末期大灾变没有触发进一步的核心形成，这种所谓的晚期薄板仍然留在地球的外部硅酸盐部分。来自晚期贴面的贵金属现在在地球上分布得很好，这个项目将遵循末日大灾变中的这种再分配。在这样做的过程中，我们将了解地幔(对流的，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，末期大灾变可能触发了我们目前所看到的板块构造的现代对流形式的开始。我们将通过观察其他古代化学特征是否与晚期贴面同时同质化来调查这一想法。这项工作的方法将是通过高精度测量一些所谓的亲铁元素的同位素比率，即W(发现于旧式灯泡和切割工具中)和Mo(用作工业润滑剂)。末次大灾变前，末次大灾变前，末期板块中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，但我们的新技术有合适的精度来检测这些。用同位素比值来追踪这一过程的强度在于，它们基本上只受混合作用的影响，而几乎不受其他地质作用的影响。通过对一系列越来越年轻的岩石中钼和钨的同位素比值的研究，我们希望追踪晚期板岩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.