Deep Mantle Recycling Revealed in Diamonds and their Mineral Inclusions (NERC Grant NE/J008583/1)

Three Published Papers; Thomson et al CMP 2014 - Origin of Sub-Lithospheric diamonds from the Juina-5 Kimberlite (Brazil): constraints from Carbon Isotopes and Inclusion Compositions http://dx.doi.org/10.1007/s00410-014-1081-8. Thomson et al Nature 2016 - Slab melting as a barrier to deep carbon subduction http://dx.doi.org/10.1038/nature16174 Burnham et al 2015 - Stable Isotope evidence for Crustal Recycling as recorded by superdeep Diamonds http://dx.doi.org/10.1016/j.epsl.2015.10.023 NERC grant abstract: Natural diamonds are formed at high pressures and temperatures deep within the Earth's interior. When diamonds form, probably from carbonate-rich fluids and melts in the mantle, they sometimes encapsulate small pieces of the minerals that occur at great depth in the Earth. These are called mineral inclusions. The diamonds are then transported from Earth's deep mantle to the surface in uncommon magmas called kimberlites. Diamonds that contain these mineral inclusions are very rare, and offer a truly unique glimpse into what is an otherwise inaccessible portion of the Earth. Some very rare inclusions provide direct samples of lithologies present in the mantle transition zone (400 - 660 km) and the lower mantle (>660 km) - these are often called superdeep diamonds. The chemistry of the inclusions along with mineral phase relations yield important information about the kinds of lithologies they originated in, and constrain the conditions of diamond formation and the depth at which kimberlite magmas form. Thus, superdeep diamonds are very important for studying the types of materials that occur in the deep Earth, for elucidating deep mantle processes, and for understanding how carbon is cycled from the surface to the mantle and back to the surface again - the deep carbon cycle. For example, some diamonds contain materials that are very similar to those occurring near the earth's surface, such as minerals akin to oceanic crust or sediments, and these often have carbon isotopic compositions akin to organic carbon - although this is a controversial subject. From this, we can conclude that surface materials can be transported to great depth, helping to constrain models of mass transfer in Earth by mantle convection. Further, by dating when the diamonds formed, for example by dating of inclusions, we can effectively place time constraints in the geodynamic processes involved in diamond formation and uplift in the mantle. Inclusion-bearing diamonds suitable for study are very hard to come by. We are very fortunate to be in possession of several large suites (over 200 inclusion-bearing diamonds in all!) of diamonds from kimberlite pipes in the famous Juina region of Brazil, a region known for its superdeep diamonds. Our previous study on diamonds from the Juina region has yielded some fascinating results, and has led to a model of material recycling beneath Brazil that we have recently published in the journal Nature and in Contributions to Mineralogy and Petrology. We now wish to extend our investigations by studying new suites of diamonds from Juina to test our current model, and to make high-pressure temperature experiments that will allow us to determine at what depths the inclusions formed and equilibrated, and will provide information needed to constrain the rates at which diamonds were transported in the solid-state mantle, possibly in a mantle plume. Here, we propose a three-year project for a comprehensive mineralogical, geochemical, isotopic and experimental investigation of these unique diamonds and their mineral inclusions.

三篇已发表论文；Thomson 等人 CMP 2014 - 深部地幔钻石的起源：Juina-5 金伯利岩（巴西）的碳同位素与包裹体组成研究约束 [http://dx.doi.org/10.1007/s00410-014-1081-8]。Thomson 等人 Nature 2016 - 岩盘熔融作为深部碳俯冲的屏障 [http://dx.doi.org/10.1038/nature16174]。Burnham 等人 2015 - 通过超深部钻石记录的地壳再循环的稳定同位素证据 [http://dx.doi.org/10.1016/j.epsl.2015.10.023]。NERC 项目摘要：天然钻石在地球深部高压高温的环境中形成。钻石形成过程中，可能由富含碳酸盐的流体和地幔中的熔融物质构成，有时会包裹住地球深部存在的矿物碎片，这些碎片被称为矿物包裹体。随后，钻石通过罕见的金伯利岩浆从地球深部地幔运送到地表。含有这些矿物包裹体的钻石极为罕见，为我们提供了一个真正独特的视角，窥探地球深部那些原本无法触及的部分。一些极为罕见的包裹体提供了直接样品，展示了地幔过渡带（400 - 660 公里）和下地幔（>660 公里）中存在的岩性，这些样品通常被称为超深部钻石。包裹体的化学成分以及矿物相关系提供了关于它们起源岩性的重要信息，并限制了钻石形成的条件和金伯利岩浆形成的深度。因此，超深部钻石对于研究深部地球中存在的物质类型、阐明深部地幔过程以及理解碳从地表到地幔，再从地幔返回地表的循环过程——深部碳循环具有重要意义。例如，一些钻石包含与地球表面附近存在的物质非常相似的成分，如类似海洋地壳或沉积物的矿物，这些物质往往具有与有机碳相似的碳同位素组成——尽管这是一个具有争议的课题。据此，我们可以得出结论，地表物质可以被运输到深处，从而有助于约束地球通过地幔对流的质量转移模型。此外，通过测定钻石形成的年代，例如通过包裹体的年代测定，我们可以有效地为涉及钻石形成和地幔抬升的地质动力学过程设置时间约束。具有研究价值的含有包裹体的钻石非常难得。我们非常幸运地拥有来自巴西著名Juina地区的金伯利岩管中的一些大型钻石套件（总计超过200颗含有包裹体的钻石！），该地区以其超深部钻石而闻名。我们之前对Juina地区钻石的研究已经取得了一些引人入胜的结果，并导致我们最近在《自然》杂志和《矿物学及岩石学贡献》期刊上发表了一个关于巴西下地幔物质循环的模型。现在，我们希望通过研究Juina地区的新一批钻石来检验我们的当前模型，并进行高压高温实验，这将使我们能够确定包裹体形成的深度和平衡状态，并提供关于钻石在固态地幔中可能在地幔柱中迁移速率的所需信息。在此，我们提议一个为期三年的项目，对这批独特的钻石及其矿物包裹体进行全面的矿物学、地球化学、同位素和实验研究。