Paper No. 2
Presentation Time: 8:25 AM


SHIREY, Steven B., Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington, DC 20015 and SHIGLEY, James E., Research Department, Gemological Institute of America, 5345 Armada Drive, Carlsbad, CA 92008,

It has been more than two decades since diamond ages have been shown to be billions of years older than their host magmas of kimberlite or lamproite. Significant advances have occurred in the analysis of diamonds and their mineral inclusions, in the understanding of diamond-forming fluids in the mantle, and in the relationship of diamonds to the deep geology of the continents and the convecting mantle. Diamonds of all types, including those that are monocrystalline and gem-quality have become one of the most important ways to study the deep Earth. Diamonds form from C-O-H-S fluids that flow through deep mantle rocks, especially in the lithosphere. These fluids transform the rocks by metasomatism while precipitating diamond. The redox state of the diamond host rocks controls the diamond forming reactions and can tie geologic processes locally and globally to specific diamonds. Throughout Earth history, diamonds formed by a multiplicity of reactions -not just one type of reaction. The formation of diamond is potentially widespread in the mantle. Past work on diamonds has suggested the initiation of subduction, tracked the transfer of material through the mantle transition zone, recorded the timing and source of ingress of fluids to the continental lithosphere, preserved carbonatitic fluids that trigger deep mantle melting, captured the redox state of the mantle, and provided samples of deep carbon and noble gasses. Future work on diamonds will establish the relative proportion of recycled vs primodial carbon, produce accurate stable isotopic fractionation factors for C and N during diamond growth, trace the pathways of carbon into the deep mantle, establish the role of highly mobile, carbonatitic magmas and regions where metal formation can leave reduced carbon ready for diamond crystallization. As one of the oldest minerals on Earth, diamond has a unique ability to show us how these processes have changed with time.