ORIGIN OF DIAMONDS IN SKENDERBEU MASSIF, MIRDITA OPHIOLITE, ALBANIA: IMPLICATIONS FROM ISOTOPES AND INCLUSION COMPOSITIONS

Here we report a new type of diamond occurrence from the Skenderbeu massif in Mirdita ophiolite. The host diamond hosting ophiolite originated from shallow mantle depth. Skenderbeu massif is considered to be a part of Jurassic Tethys oceanic domain in West Albania, a tectonic setting distinct from that of other currently discovered diamonds. More than twenty microdiamonds sourced from the Skenderbeu massif, which contain micro- and nanoinclusions. These inclusions consist of a nickel-based manganese-cobalt alloy, coexisting with nanocrystals (20 x 20 nm) of calcium silicate in orthorhombic perovskite structure and fluid. Qualitative analysis of the fluid shows that it is composed of C-Si-Ca-Cl-O-Mn, Mg and K. The fluid coexisting with that alloy proves that the diamonds in Skenderbeu massif are indigenous rather than synthetic diamond. Diamond crystallized from a redox-sensitive Ni-Mn-Co alloy melt in the deep mantle at least in the diamond stability field. All of the diamonds (n= 12) are composed of isotopically light carbon, and have a unimodal distribution heavily skewed towards δ¹³C_PDB ~ −25‰. The light carbon isotopic compositions are interpreted as evidence of diamond growth from organic carbon added to the subducted oceanic crust. The diamonds (n= 8) contain 37.0 – 286.4 ppm nitrogen (N) and show an exceptionally large range in δ¹⁵N_Air (-12.9‰ to +25.5‰). These characteristics mentioned above suggests that the Skenderbeu diamonds nucleated and grew from a C-saturated nickel–manganese–cobalt-rich melt in a highly reduced environment. Nitrogen isotopic composition data is as evidence for nitrogen isotope heterogeneity in Skenderbeu diamond-forming fluids/melts on localized scales. We suggest here that the Ni-Mn-Co alloy originated from the melting of the Mn-nodules in the subducted oceanic crust. High-density fluids (HDFs) released from the subducted oceanic crust contain Cl+O+Si+K+Ca+CO₃^2-+H₂O. The HDFs react with the alloy melt and the alloy melt reduces the HDFs thus releasing carbon which is dissolved in the metal alloy. Diamond precipitated from the C supersaturated melt when PT changed. It is suggested that subsequent rapid upward transport in channeled networks may explain the formation and preservation of Skenderbeu diamonds.