Super-deep diamonds from kimberlites in the Juina area, Mato Grosso State, Brazil

Abstract

Abstract One thousand three hundred sixty-five diamonds from seven newly discovered kimberlitic pipes in the Juina area were comprehensively studied. These diamonds, like the ones from previously studied Juina placer deposits, are very homogeneous in their morphology and optical properties. Two diamond populations exist in the Pandrea pipes: the major population with a highly aggregated nitrogen impurity (% NB = 75–100%), and a secondary population with a moderately aggregated nitrogen impurity (% NB = 20–65%), while only one major population is present in the diamonds from placers. The diamonds from the pipes have a permanent, relatively high hydrogen impurity concentration. Among the mineral inclusions in diamonds from the Juina pipes, ferropericlase is predominant; chrome spinel, picroilmenite, Mn-ilmenite, MgCaSi-‘perovskite’ phase, rutile, sulphide, native iron, and iron-oxides were also identified. Most of the inclusions belong to the lower-mantle paragenesis; some (rutile and sulphide) are of eclogitic paragenesis. Mineral inclusions in diamonds from kimberlitic pipes are different in composition from the same minerals in placer diamonds. Both kimberlitic and placer diamonds belong to the same carbon isotopic population, but have differences in the δ13C distribution and were probably formed from different local carbon sources. These data indicate that diamonds from both groups, kimberlites and placer deposits, in the Juina area, belong to the same genetic population with most of the stones originating within the super-deep conditions. However, there are differences between these two groups, which indicate that besides the known Pandrea pipes which may have partly supplied diamonds to the placer deposits, there may be other, still unknown primary sources of diamonds in the Juina area.