Diamonds from ultrahigh pressure terranes
a key for understanding crustal carbon reservoir in Earth’s interior

Prof. Larissa F. Dobrzhinetskaya

Institute of Geophysics and Planetary Physics, Department of Earth Sciences, University of California at Riverside, USA CA-92521

During last 25 years a new non-volcanic type of diamonds was discovered in Kazakhstan, China, Norway, Germany and Greece within metasedimentary rocks occurred in orogenic belts formed at convergent plate boundaries in Paleozoic-Mezosoic (480-250 Ma) time. These diamonds as small as 1–80 µm exhibit imperfect morphologies and contain abundant fluid and crystalline nanometric size inclusions indicating mixture of both crustal and mantle components. The nitrogen impurities recorded in these diamonds suggest that all of them belong to the type 1b-1aA, implying a short residence time at high temperature (~900–1,100°C) of ca 5 Ma. The characteristics of single N defects (1b component) and N pairs (1aA components) in the orogenic belts diamonds distinguish them from other nitrogen-bearing diamonds of kimberlitic sources (type 1aAB), having much longer residence time in the Earth’s interior. The diamonds from UHPM terranes occurred inside of multi-compositional pockets where they are very frequently accompanied by hydrous phases, phosphates, and oxides. A σ13C value and the nitrogen content are widely used to constrain conditions of formation and the source of media (fluid/melt) from which diamonds are crystallized. In the Kazakhstan diamond deposit (Kokchetav massif) the diamonds are characterized by σ 13C=10.57‰ and N =11,150-2,500 ppm, whereas the diamonds from the German site (Erzgebirge massif) have σ 13C= 17‰ to 27‰ and N =100–4,647 ppm. The ranges of σ 13C suggest that the Kazakhstan diamonds were formed from mixed “crustal-mantle” carbon reservoirs, whereas diamonds from the German site were crystallized from unmixed ‘‘crustal’’ carbon reservoir, either of organic or inorganic origin. Compositional studies of nanometric fluid inclusions suggest that the orogenic belts diamonds crystallized from a supercritical C-O-H fluid in deep subduction zone. During deep subduction up to ~150-210 km in some collisional orogens the diamond-bearing slab has been interacted with mantle wedge chemical environments resumed in the mixture of both geochemical and carbon reservoirs (e.g., Kazakhstan). In the case of the German site, the primary diamond-forming cabon reservoir has remained unmixed, and the diamond themselves contain nanoscale inclusions originated mostly from the continental crust material. Given that these diamonds were crystallized at the same depth as diamonds from the Kazakhstan site we assume that the Erzgebirge diamond-bearing slab was “geochemically shielded” having no interaction with the mantle material. The integrated studies of diamonds from UHPM terranes provide information that strongly supports the concept of diamond crystallization from a COH-supercritical fluid; that was successfully reproduced by laboratory experiments. The studies also emphasizes that the ‘‘crustal’’ carbon may be delivered at list to depth up to 210 km through the subduction of continental (and oceanic) sediments. These observations and conclusions will serve as a foundation for future calculations of budget of the “crustal” carbon that have formed deep reservoirs within the Earth’s interior during Paleozoic-Mesozoic continental margins activities.