Mineral inclusions in diamonds: associations and chemical distinctions around the 670-km discontinuity

Three new mineral associations have been discovered within diamonds from the Juina district of Brazil. These include a previously unrecorded NaAl-(Mg, Fe)SiO3 phase associated with ferroperidase and the tetragonal almandine pyrope phase, TAPP. Also reported are an association of corundum with aluminous-pyroxene and an olivine composition phase associated with ferroperidase. The minerals in each association often occurred within the same diamonds in addition to being recovered from individual diamonds in different combinations. High-pressure experimental data indicate that these associations formed at different depths within a region ca. 60 km on either side of the upper-mantle/lower-mantle boundary. Mineral compositions show that for the regions sampled, the deep transition zone and lower mantle are chemically distinct and inhomogeneous. Importantly, in the shallow lower mantle, Al is not solely accommodated within perovskite-structured (Mg, Fe)SiO3 as some recent experimental studies have suggested.