The influence of lithospheric structure and composition on the distribution of CO2-rich intraplate volcanism and REE mineralisation

地球上的大部分火山活动发生在构造板块的发散和会聚边界上。虽然板块内大陆环境中的火成岩活动不太常见，但这些岩浆掌握着地球外部刚性壳(岩石圈)的演化以及具有重要商业意义的自然资源的产生和分布的基本线索，包括对可再生能源和其他基础设施至关重要的稀土元素(REE)和关键金属(例如Nb和Ti)。大陆板内岩浆活动的类型由岩石圈的厚度和成分以及岩石圈和下地幔中的温度决定。板内玄武岩是在较薄(60-70公里)岩石圈之下的软流圈中部分熔融产生的，而碳酸盐岩和与之伴生的碱性火成岩则是通过较厚(约70-120公里)的岩石圈侵位的，它们是世界上大多数稀土矿床的所在地。这与产于岩石圈厚度超过150公里的前寒武纪克拉通中的含钻石金伯利岩和钾镁岩形成鲜明对比。碳酸盐岩杂岩见于大陆裂谷带、大型火成岩省和同-后碰撞带。以前对它们成因的研究主要集中在它们与地表地质的关系上，但在这里，我们通过考察它们的分布和深部岩石圈结构来进行创新。我们的工作是及时的，因为：1.随着世界各地地震台站数量的迅速增长，地震层析成像现在能够以比以前更高的分辨率绘制大多数大陆地区之下的岩石圈结构的横向变化。岩石圈结构的高分辨率地球物理模型，以及从不同年龄的全球碳酸盐岩杂岩中收集的样品，使人们能够对它们在空间和时间上的分布有新的、独特的见解。稀土是新兴清洁、绿色技术、电子设备和炼油催化剂产品的必需品，满足日益增长的需求是一项严峻的全球挑战。全球95%以上的可用稀土来自白云鄂博矿(中国)，这引发了人们对欧洲供应安全的担忧(例如欧洲稀土项目http://www.eurare.eu/about.html))，并强调有必要确定未来潜在矿藏的来源。这种勘探对长期规划和增长至关重要，但欧洲目前没有供应稀土的矿山，尽管有一些地区的地质经常与稀土矿化有关，包括在格陵兰岛西南部、芬兰和斯堪的纳维亚发现的碱性火成岩。我们研究的主要目标是量化岩石圈厚度与碳酸岩杂岩及其相关的REE、Ti、Nb和Ta矿床的全球分布之间的关系，并评估这种变化在地球演化过程中的变化。这涉及到地球系统内的大规模、复杂的相互作用。我们在全球和区域范围内进行了初步研究，重点是非洲东部和美国西部的S，研究发现：(I)大陆岩浆类型的分布与岩石圈厚度之间存在相关性；(Ii)碳酸盐岩杂岩与古克拉通边缘之间存在强烈的关系；(Iii)显生宙碳酸盐岩岩浆活动往往位于比前寒武纪更薄的岩石圈上。我们寻求在试点研究的基础上，提高地球物理数据的分辨率，将分析扩大到所有大陆，并汇编不同时代的碳酸盐岩杂岩和相关碱性火成岩及其稀土矿床的综合地球化学数据集。利用对地球物理和地球化学数据的深入统计分析，我们将量化这些相关性，确定它们如何随地质时间变化，建立岩石和矿床位置的概率预测模型，并增进我们对岩石圈演化的了解。

Most of the Earth's volcanism occurs at the diverging and converging boundaries of tectonic plates. While igneous activity is less common in intraplate continental settings, these magmas hold essential clues on the evolution of Earth's outer rigid shell (lithosphere) and the generation and distribution of commercially significant natural resources, including rare earth elements (REE) and critical metals (e.g. Nb & Ti), essential for renewable energy and other infrastructure.The type of continental intraplate magmatism is governed by the lithospheric thickness and composition and temperature in the lithosphere and underlying mantle. Intraplate basalts are produced by partial melting in the asthenosphere beneath thin (<60-70 km) lithosphere, whereas carbonatites and associated alkaline igneous rocks, which host most of the world's REE deposits, are emplaced through thicker (~70-120 km) lithosphere. This contrasts with diamondiferous kimberlites and lamproites that occur in Precambrian cratons, with lithosphere over 150 km thick.Carbonatite complexes are found in continental rift zones, Large Igneous Provinces and syn- to post-collisional zones. Previous studies of their origins have largely focused on their relationship with surface geology but here we innovate by examining their distribution with deep lithospheric structure. Our work is timely because:1. It follows the rapid recent growth in the number of seismic stations around the world, which now permits seismic tomography to map lateral variations in lithospheric structure at higher resolution beneath most continental areas than previously possible. High-resolution geophysical models of lithospheric structure together with access to samples collected from global carbonatite complexes of different ages enables novel, unique insights into their distribution in space and time.2. Keeping up with increasing demand to supply REEs, essential for products in emerging clean, green technologies, electronic devices and petroleum-refining catalysts, presents a serious global challenge. Over 95% of the world's usable REEs are produced from the Bayan Obo mine (China), raising concerns over the security of supply for Europe (e.g. EURARE project http://www.eurare.eu/about.html) and highlighting the need to identify sources of potential future deposits. This exploration is essential for long-term planning and growth, but Europe currently has no mines supplying REEs, although there are a number of areas with geology often associated with REE mineralisation, including alkaline igneous rocks found in south-west Greenland, Finland and Scandinavia.The main goal of our research is to quantify the relationship between lithospheric thickness and the global distribution of carbonatite complexes with their associated REE, Ti, Nb and Ta deposits, and assess how this has varied during Earth's evolution. This relates to large-scale, complex interactions within the Earth system. We have conducted pilot studies at global and regional scales, focused on E Africa and western S America, which revealed: (i) a correlation between the distribution of continental magma type and lithospheric thickness; (ii) a strong relationship between carbonatite complexes and the margins of ancient cratons; (iii) Phanerozoic carbonatite magmatism is often located on thinner lithosphere than its Precambrian counterpart. We seek to build on the pilot study by improving the resolution of the geophysical data, expanding the analysis to all the continents, and compiling comprehensive geochemical datasets for carbonatite complexes and associated alkaline igneous rocks of different ages together with their REE deposits. Using in-depth statistical analysis of the geophysical and geochemical data, we will quantify the correlations, establish how they varied over geological time, build a probabilistic predictive model for the location of the rocks and deposits, and advance our knowledge of lithospheric evolution