Windows into the Deep: An Investigation of Ultradeep Mineral Inclusions in Natural Diamonds

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Natural diamonds are formed at high pressures and temperatures deep within the Earth's interior. When diamonds form, probably from carbonate-rich fluids 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. They are very important for studying the types of materials that occur in the deep Earth. For example, some diamonds contain materials that are very similar to those occuring near the earth's surface. 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. A longstanding problem in Earth sciences is whether convection is a whole-mantle process or whether there are discrete convecting regimes in the mantle. If diamonds can be identified that have mineral inclusions that formed in Earth's lower mantle, this can give considerable support to a whole-mantle convection model. Some very rare inclusions seem to provide direct samples of lithologies present in the mantle transition zone (400 / 660 km) and possibly the lower mantle (>660 km). The chemistry of the inclusions along with mineral phase relations yield important information about their mantle protoliths, and constrain the conditions of diamond formation and the depth at which kimberlite magmas form. We have been fortunate to obtain a large suite of diamonds from a kimberlite pipe in Brazil, many of which contain mineral inclusions. Our preliminary work on these inclusions has shown that some have unique chemistry indicating a very deep origin - perhaps in the lower mantle. Here, we propose a two-year project for a comprehensive geochemical, isotopic and experimental investigation of these unique diamond inclusions. By analyzing their chemical and isotopic composition in detail we can obtain important information about how the diamonds formed and about the chemistry of the silicate and oxide materials they formed within. Further, by understanding mineral phase relations through high pressure experiments we can place constraints on the depths of origin of the mantle inclusions.