The origin and evolution of cratons: Age and chemical constraints from the lithospheric root beneath West Greenland.

就像冰山一样，大陆的主体隐藏在“地幔根”深处。在古老的地壳岩石（大约25亿年的历史）下，被称为克拉通，许多这些根延伸到很深的深度（至少250公里，也许400公里），非常古老，是世界上宝石钻石供应的宿主。可以说，这些克拉通地幔根最有趣的方面是，它们似乎有助于大陆地壳的长期生存。要了解克拉通及其根形成和稳定的机制，需要了解各种克拉通根的总体组成及其形成年龄。这是很难做到的，因为树根的样本只在某些地区由深层衍生的火山岩喷发出来，而且许多样本太小，无法获得对其总体成分的代表性估计或获得年龄信息。非洲南部的Kaapvaal克拉通似乎具有不同寻常的组成，因此我们需要一个克拉通岩石圈的替代基准数据集，以提高我们对典型克拉通形成和演化的认识。暴露在格陵兰西南的北大西洋克拉通及其周围的变形克拉通地壳，被深层衍生的火山岩侵入，这些火山岩携带着大量的地幔根碎片。利用已发表的不同压力下地幔熔融实验数据，以及对不同压力下熔融过程中与固体残留物具有不同亲和力的元素的分析，我们计划限制在该地区不同部分下形成地幔根的熔融深度。这将使我们能够检验一个模型，该模型表明克拉通的根是由更古老的海洋板块的俯冲堆积形成的。对地幔碎片的矿物分析将使我们能够评估它们产生的深度，从而观察不同区域下方根部的厚度。我们还将在矿物和大块岩石上使用放射性衰变系统187re -1870和176Lu-176Hf，以确定熔体提取事件的年龄，并试图确定与导致更大克拉通破碎的构造事件相关的任何后续根扰动的年龄。通过这种方法，我们可以了解不同时代地壳下深层地幔根的年龄和深度。这不仅能让我们了解克拉通是如何形成的，还能让我们了解它们是如何受到主要构造事件的影响的，以及它们为什么有时会分裂。最后，在侵入克拉通和克拉通周围地区的火山岩中发现了钻石，这些火山岩受到构造事件的严重影响。了解克拉通根部的年龄、深度和组成如何横向变化，也将提高我们对克拉通内部和周围根部钻石分布的理解，以及最终是什么过程导致了它们的形成或破坏。

Like an iceberg, the main volume of a continent is hidden at depth in 'mantle roots'. Beneath ancient crustal rocks (>2.5 billion years old), known as cratons, many of these roots extend to great depth (at least 250 km and perhaps 400 km), are very ancient and are host to the worlds supply of gem diamonds. Arguably the most interesting aspect of these cratonic mantle roots is that they seem to assist in the long-term survival of continental crust. To understand the mechanisms by which cratons and their roots are formed and stabilised requires information on the bulk composition of various cratonic roots and their age of formation. This is difficult to achieve because samples of the roots are only erupted by deeply derived volcanic rocks in certain areas and many samples are too small to obtain representative estimates of their bulk compositions or obtain age information. The one craton where addressing these issues has been possible in a detailed way, the Kaapvaal craton, S. Africa, appears to have an unusual composition and hence we are in need of an alternative benchmark dataset for cratonic lithosphere, with which to improve our knowledge of how typical cratons form and evolve. The North Atlantic Craton, exposed in S.W. Greenland, together with its surrounding areas of deformed cratonic crust, is intruded by deeply derived volcanic rocks carrying an exceptional inventory of large fragments of the mantle root beneath. Using published data from the experimental melting of mantle at different pressures, together with the analysis of elements that have differing affinities for the melt compared to the solid residue during melting at different pressures, we plan to constrain the depth of melting that formed the mantle root beneath the differing parts of this region. This will enable us to test a model that suggests that cratonic roots are formed by the subduction stacking of older oceanic plates. Mineral analysis of the mantle fragments will allow us to evaluate the depths from which they were derived and hence look at the thickness of the root beneath different areas. We will also use the radiometric decay systems 187Re-1870s and 176Lu-176Hf, on both minerals and the bulk rock, to determine the age of the melt extraction event and to try to determine the age of any subsequent disturbance of the root associated with tectonic episodes responsible for the fragmentation of a much larger craton. In this way we can obtain an understanding of the age and depth of the deep mantle root beneath crustal regions of different ages. This will provide us with a picture of not only how cratons are formed, but how they are affected by major tectonic events and why they sometimes breakup. Lastly, diamonds have been found in the volcanic rocks intruding both the craton and the region surrounding the craton, that has been heavily influenced by tectonic events. Understanding how the age, depth and composition of the cratonic root beneath region changes laterally will also improve our understanding of how diamonds are distributed in the roots in and around cratons and ultimately what processes by cause their formation or destruction.