A Specific Family of Yellow Cuboid Diamonds from the Udachnaya and Yubileynaya Kimberlite Pipes (Siberian Craton)

Abstract Yellow cuboid diamonds are widespread in alluvial placers of the northeastern Siberian craton, but are extremely rare in kimberlite pipes. Here we present the first systematic study of 41 cuboid diamonds from the Udachnaya (n = 23) and Yubileynaya (n = 18) kimberlite pipes of the Siberian craton. The diamonds exhibit variable resorption features with overall cuboid growth patterns and belong predominantly to the rare Ib–IaA type, containing C, A, X, and Y nitrogen defects in different proportions. Udachnaya cuboid diamonds show higher nitrogen contents (~1000–1750 ppm) and aggregation states (%A = ~60–95), with some cores containing up to 15% of their nitrogen in the form of B-defects, whereas Yubileynaya cuboid diamonds are dominated by low-aggregated domains (Ntot ~50–600 ppm, % A = 0–40%) alongside a minor high-aggregated group. Carbon isotope compositions of Yubileynaya diamonds (–21.1 to –5.8‰) overlap with those of northeastern placer diamonds and cover most of the range of global eclogitic diamonds, while Udachnaya diamonds show a narrow mantle-like range (–5.6 to –2.3‰). Microinclusion compositions in the studied diamonds are dominated by SiO2, Al2O3, CaO and K2O and belong to the silicic to low-Mg carbonatitic series, consistent with an eclogitic source for the diamond-forming fluids. The strong resemblance of Yubileynaya (and, to a lesser extent, Udachnaya) yellow cuboid diamonds to those from northeastern Siberian placers suggests that kimberlites may be the primary sources of these alluvial diamonds and highlights the key role of subduction processes in their formation.