Mantle Metasomatism and the Origin of Diamonds

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The amount of water in the mantle of the Earth may exceed that in all the oceans, lakes, and rivers of the world. The finding of water as hydroxyl (OH) in mantle minerals in the 90's was highly significant, but these minerals may have been altered upon ascent from depths to the surface in the kimberlites, also the host for diamonds. However, a recent discovery of water in the mineral 'ringwoodite' (Mg2SiO4) included in a diamond has demonstrated that water can reside within minerals deep in the Earth's mantle for a long time. Mineral inclusions in diamonds are thought to have remained pristine since their encapsulation hundreds of miles deep in the Earth's mantle, where diamonds are formed. They are the only samples we have of the 100 mile-deep portions of the Earth and are the subject of this proposal. In particular, it is the fluids in the mantle that formed the diamonds, the water-bearing metasomatic fluids, which can reveal the conditions responsible for formation of diamonds as well as their inclusions. The main objectives of the proposed research are several, all associated with understanding the metasomatic, hydrothermal, carbonatitic system that carries the signatures of the super-critical fluids that constitute the 'hidden reservoir' of water within the Earth. The mineral inclusions in diamonds will be chosen from the 100 diamondiferous xenoliths, upon which this team has previously studied and published. Specifically, the study will focus on the chemistry of diamond inclusions, particularly their volatile contents and isotope compositions (e.g., OH and D/H values in silicates; Cl-F-S in sulfides). This renewal period will capitalize on this team's ability to effectively polish diamonds, exposing their numerous mineral inclusions simultaneously on one surface. With cathodoluminescence imaging of these several inclusions, it will be possible to sample them for further studies of their relative ages and possibly recreate the chemistry of the changing metasomatic fluids during the individual diamond's lifetime of growth, covering perhaps 10's of millions of years.