Deep Mantle Recycling Revealed in Diamonds and their Mineral Inclusions

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.

Who will benefit:

The research proposed here will lead to a better understanding of the origin of diamonds and the fluids and melts from which they crystallize, deep mantle processes, deep mantle recycling, and the deep carbon cycle. This fundamental knowledge will primarily interest academics concerned with the evolution of the Earth and other terrestrial planets - petrologists, geochemists, geophysicists and geodynamicists - who would gain deeper insight into the physical and chemical properties of Earth's interior. Other academics who will benefit are those in the mineral exploration community, as results obtained from studying diamonds and their inclusions can sometimes be used as exploration tools.

What will be done.

Academic beneficiaries: Key outputs will be publications in scientific journals and presentation of the work at international conferences. We will run a teo-day workshop in year 2 of the project that will focus on understanding the diamond formation processes, deep recycling of subducted material, and the deep carbon cycle. We will bring together experts from the UK, Europe and USA to foster exchange of experimental, theoretical and observational results and discussion of their interpretation.

Undergraduate Students: We will run a one-day workshop in year 2 called Diamond Day specially aimed at year 3 and year 4 undergraduate students at U. Bristol (~ 30). This forum will provide an opportunity for these young scientists in training to get a feel for how research is done and disseminated in the scientific community.

General public: We will design a web-page targeted at general audience and students that will provide information about research into the deep carbon cycle and diamond generation. We will also develop a number of power-point presentations that we take present at local secondary schools as part of our Departmental outreach program.