Diamonds Through Time and Space: Unique Windows on Mantle Evolution

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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.