3. Lower-Mantle Mineral Associations

There are three major sources of information about the composition of the lower mantle: high P–T experiments, theoretical calculations, and geological observations. Experimental data, based on the use of diamond-anvil cell technique (DAC), and theoretical calculations demonstrate that silicates, occurring in the upper mantle and the transition zone, are replaced by predominantly perovskitic assemblage in the lower mantle. Depending on the starting substrate composition, two mineral associations should occur at pressures corresponding to the lower-mantle conditions: ultramafic (bridgmanite + CaSi-perovskite + ferropericlase) and mafic (bridgmanite + CaSi-perovskite + ferropericlase + silica + Al-phase). Both iron-containing lower-mantle minerals, bridgmanite and ferropericlase, should be magnesium-rich. In recent decades, lower-mantle minerals were found as inclusions in diamonds from Brazil, Guinea, Canada, Australia and South Africa. They confirm the presence of ultramafic, mafic and carbonatitic mineral associations. Geological samples differ notably from the lower mantle compositions suggested on the basis of experimental and theoretical data for the pyrolitic composition. First, ferropericlase is the most common in the lower-mantle ultramafic association (averaging 55.6%), while bridgmanite comprises only 7.5%, about ten times lower than has been suggested (c. 70–74%) in the lower mantle. Second, silica inclusions were identified in all sets of lower-mantle minerals observed in diamond from all regions and areas. Third, wide variations in ferropericlase compositions, reaching an iron index of up to fe = 0.64 were observed. Minerals from the ultramafic association overwhelmingly predominate in the lower mantle samples; only two samples of mafic mineral phases, phase Egg and δ−AlOOH are found to date.