Diamonds Through Time and Space: Unique Windows on Mantle Evolution

该项目的目标是分析钻石及其所包含的地幔矿物的微观包裹体，以调查地球上的深部火成岩过程。钻石是已知的最深、最古老、最坚固的地幔样品容器。这确保了包裹在钻石中并由其宿主金伯利岩输送到地表的地幔矿物处于原始状态。正在研究的钻石系列在空间、时间和成分上都不同，形成于三个根本不同的地球动力学环境中，年龄从古老到年轻，从3000万年到1亿年不等。巴西9300万年前的Juina 5和Collier 4金伯利岩所携带的钻石来自对流地幔，来自过渡带或略低于过渡带，是最年轻的。在圭亚那21亿年前的达钦科马提岩中喷发的钻石来自对流地幔，来自汇聚的边缘环境，可能与其宿主岩浆的年龄相似。来自津巴布韦5.38亿年前的Murowa和Sese金伯利岩的钻石来自非洲大陆之下的岩石圈地幔龙骨，它们是在地幔龙骨稳定期间侵位的，可能超过3000万年。这项研究的目标有三个方面：1)了解钻石的形成如何与地幔对流、俯冲和大陆稳定等更广泛的地球动力学过程有关；2)利用钻石中保存独特的矿物颗粒，确定其在再循环地壳成分、地幔端元或钻石结晶所来自的熔体/流体中的来源；以及3)利用包裹体的成分、水含量和年龄开始拼凑出地球深部地幔挥发性分布的图景，摆脱岩浆分异带来的不确定性。在此过程中，我们将检验与地球动力学环境相关的三个重要假说：1)过渡带异常富含水和回收组分；2)钻石可以在俯冲环境中从回收组分中形成；3)最早、最贫乏的地幔龙骨可以在不交代或重新受精的情况下直接形成钻石。进行了单一硫化物包裹体Re-Os同位素测年，以确定硫化物的年龄和初始Os同位素组成。成像技术(阴极发光和扫描电子显微镜)正被用来揭示金刚石的生长历史和包裹体与金刚石母体的关系。光谱方法(红外和拉曼)正被用来限制钻石的时间-温度历史，并识别新的包裹体矿物。目前正在对寄宿钻石的生长带进行碳和氮同位素原位工作，以查明钻石流体的来源。相关的硅酸盐包裹体，如石榴石、单斜辉石、橄榄石和深部地幔相，正在进行水分含量分析，如有必要，还会分析微量元素。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相同标本的部分进行补充研究。这个项目具有很强的教育意义，因为史密森学会的钻石收藏品是世界上参观人数最多的博物馆展览。展览缺乏关于钻石地质的基本信息，如钻石年龄、钻石是如何形成的、钻石揭示了大陆深根的哪些信息，以及总体上的地幔。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。这些措施包括在展出的展品中增加钻石地质学内容，创建突出钻石地质学的新展品，向广受欢迎的史密森学会网站提供钻石地质学内容，以及与史密森学会教育部工作人员合作创建可供动手和授课使用的教育材料。该项目还支持对华盛顿卡内基研究所博士后研究员的培训。

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.