First find of polycrystalline diamond in ultrahigh-pressure metamorphic terrane of Erzgebirge, Germany

This article describes polycrystalline diamond from the Erzgebirge ultrahigh-pressure metamorphic (UHPM) terrane of the Bohemian massif related to the Variscan (340 Ma) continentcontinent collision. The foils polycrystalline diamond included in zircon were prepared with focused ion beam (FIB) and studied with transmission electron microscopy (TEM). Each domain of the polycrystalline diamond consisted of a series of diamond monocrystals (from 1 to 4 up to 15) containing numerous nanometric fluid and crystalline inclusions of SiO2, CaCO3, KCl and KAlSi3O8. The integrated chemistry of the nanoinclusions suggests that the polycrystalline diamond was crystallized from a mixture of two end-members, which includes a carbon-rich hydrous-silicic fluid containing Si, Al, K and a hydrous-saline fluid rich in Cl, K, Na. Traces of carbonate, sulphate and phosphate coupled with the presence of metallic cations such as Fe, Mg, Ti, Cr, Co, Zn, Zr, Pb, As and Hg found in the diamond are coherent with the local geochemical diversity of the host metasedimentary rocks. The oxidation state of the diamond-forming fluid was close to CCO buffer, for example, fO2 = -6 log units, which is at the upper limit of the diamond stability region. The presence of dislocation arrays indicating low-angle grain boundaries together with zig-zag features or kinks at diamond grain boundaries reflects rapid crystallization of the polycrystalline diamond. Triangular fluid pockets with steps at the surface related to {110} and {111} planes of diamond could promote the formation of CH and COH bonds that probably could facilitate the process of diamond graphitization. The microstructural features of polycrystalline diamond from the Erzgebirge UHPM terrane as well as their undisputed formation from crustal/organic carbon can be used for better understanding of the origin of any polycrystalline diamond formation from both kimberlitic and alluvial sources, including enigmatic polycrystalline black diamond commonly called carbonado.