Diamonds Through Time and Space: Unique Windows on Mantle Evolution

该项目的目的是分析钻石及其包含的地幔矿物的微观包裹体，以研究地球上的深部火成岩过程。钻石是已知的最深层、最古老、最坚固的地幔样本容器。这确保了包裹在钻石中的地幔矿物，并被它们的宿主金伯利岩运送到地表，处于原始状态。所研究的这组钻石在空间、时间和成分上都是不同的，它们形成于三种完全不同的地球动力学环境中，年龄从30亿年到1亿年不等。巴西有9300万年历史的Juina 5和Collier 4金伯利岩携带的钻石来自对流地幔，来自过渡带或正下方，是最年轻的。在圭亚那达钦科马提岩中喷发的钻石来自于地幔的对流层，形成于一个辐合的边缘环境，可能与它们的寄主岩浆年龄相近。津巴布韦的Murowa和Sese金伯利岩中有5.38亿年历史的钻石来自大陆下方的岩石圈地幔龙骨，在地幔龙骨稳定期间被放置，可能有30亿年以上的历史。研究的目标有三个：1)了解钻石形成与地幔对流、俯冲和大陆稳定等更广泛的地球动力学过程之间的关系；2)利用钻石中保存独特的矿物颗粒来确定其来源，这些矿物颗粒来自于再循环的地壳成分、地幔端元或钻石结晶的熔体/流体；3)利用包裹体的组成、含水量和年龄，开始拼凑出一幅不受岩浆分选带来的不确定性影响的地球深部地幔挥发物分布图。在此过程中，我们将测试与地球动力学背景相关的三个重要假设：1)过渡带异常富含水和循环成分；2)钻石可以在俯冲环境中由循环成分形成；3)最早、最枯竭的地幔龙骨可以直接形成钻石，而无需交代或再肥沃。单硫化物包裹体、Re-Os同位素定年法测定了硫化物的年龄和初始Os同位素组成。利用成像技术（阴极发光和扫描电镜）揭示了金刚石的生长历史和包裹体与金刚石基体的关系。光谱方法（红外和拉曼）被用来限制钻石的时间-温度历史和识别新的包裹体矿物。目前正在对寄主钻石的生长带进行原位碳和氮同位素工作，以确定钻石流体的来源。相关的硅酸盐包裹体，如石榴石、斜辉石、橄榄石和深地幔相，正在分析其含水量，必要时还分析微量元素。我们来自英国布里斯托尔大学和巴西巴西利亚大学的合作者正在对同一标本的部分进行补充研究。这个项目有很强的教育成分，因为史密森学会的钻石收藏是世界上参观人数最多的博物馆展览。这次展览缺乏关于钻石的基本地质信息，比如钻石的年龄、钻石的形成方式、钻石揭示了大陆的深层根源，以及地幔的一般情况。我们正在与史密森学会合作，以多种方式向广大公众提供这个钻石项目的科学研究成果。这些措施包括将钻石地质学内容添加到展出的展品中，创建突出钻石地质学的新展品，向广泛浏览的史密森尼网站提供钻石地质学内容，并与史密森尼教育部门的工作人员合作，为实践和讲座提供教育材料。该项目还支持在华盛顿卡内基研究所培训一名博士后研究员。

The objective of this project is to analyze diamonds and the microscopic inclusions of mantle minerals that they contain to investigate the deep igneous processes on Earth. Diamond is the deepest-derived, oldest, and most robust container of mantle samples known. This ensures that mantle minerals encased in diamond and transported to the surface by their host kimberlite are in pristine condition. The suite of diamonds under study are diverse in space, time, and composition, forming in three fundamentally different geodynamic settings with ages ranging from as old as 3000 million years to as young as 100 million years. Diamonds carried by the 93 million year old Juina 5 and Collier 4 kimberlites, Brazil are from the convecting mantle, were derived from the transition zone or just below, and are the youngest. Diamonds erupted in the 2100 million year old Dachine komatiite, Guyana are from the convecting mantle, were derived from a convergent margin setting, and are likely of similar age to their host magma. Diamonds from the 538 million year old Murowa and Sese kimberlites, Zimbabwe are from the lithospheric mantle keel beneath the continent, were emplaced during mantle keel stabilization and are likely more that 3000 million years old. The goals of the research are threefold: 1) to understand how diamond formation is related to the broader geodynamic processes of mantle convection, subduction, and continent stabilization; 2) to use the uniquely preserved mineral grains in diamonds to identify their sources in recycled crustal components, mantle endmembers, or the melts/fluids from which the diamonds crystallize; and 3) to use the composition, water content and ages of inclusions to begin to assemble a picture of mantle volatile distribution in the Earth's deep interior free from the uncertainties imparted by magmatic differentiation. In doing so, we will be testing the three important hypotheses related to geodynamic setting: 1) that the transition zone is anomalously water and recycled-component rich, 2) that diamonds can form in subduction settings from recycled components, and 3) that the earliest, most depleted mantle keels could have formed diamonds directly without metasomatism or re-fertilization. Single sulfide inclusion, Re-Os isotopic dating is being carried out to determine the age and the initial Os isotopic composition of the sulfides. Imaging techniques (cathodoluminescence and scanning electron microscopy) are being used to reveal diamond growth history and the relationship of inclusion to diamond host. Spectroscopic methods (infrared and Raman) are being used to constrain diamond time-temperature history and identify new inclusion minerals. In-situ carbon and nitrogen isotopic work are being carried out on the hosting diamond with respect to its growth zones to identify the sources of diamond fluids. Associated silicate inclusions such as garnet, clinopyroxene, olivine, and deep mantle phases are being analyzed for water content and if necessary, trace elements. Our collaborators from the University of Bristol, UK and the University of Brasilia, Brazil are working on supplemental studies on the parts of the same specimens. This project has a strong educational component as the diamond collection at the Smithsonian Institution is the most visited museum exhibit in the world. The exhibit lacks information on basic diamond geology such as diamond ages, how diamonds form, what diamonds reveal about the deep roots of continents, and the mantle in general. We are working with the Smithsonian Institution to provide the scientific research findings from this diamond project to a large cross section of the lay public in multiple ways. These include adding diamond geology content to the exhibits on display, creating new exhibits highlighting diamond geology, providing diamond geology content to the widely-viewed Smithsonian website, and working with the Smithsonian Education Department staff to create educational materials for hands-on and lecture use. The project is also supporting the training of a postdoctoral researcher at the Carnegie Institution of Washington.