Evidence for phase transitions in mineral inclusions in superdeep diamonds of the Sao Luiz deposit (Brazil)

Evidence for phase transitions in mineral inclusions in superdeep diamonds of alluvial placers in the Sao Luiz River deposits (Brazil) is obtained by the electron backscatter diffraction technique. It has been shown that the crystal structure of superdeep diamonds is significantly deformed around inclusions of MgSi-, CaSi-, and CaTiSi-perovskites, SiO₂ (stishovite?), and Mg₂SiO₄ (ringwoodite?). On the contrary, significant deformations around inclusions of olivine, ferropericlase, and majoritic garnet are not detected. The absence of deformation near these minerals reveals the lack of phase transitions with dramatic volume changes. The present study suggests that the formation of superdeep diamonds proceeds at different levels of the sublithospheric mantle, transition zone, and lower mantle.