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Major rock-forming minerals of the upper-mantle juvenile ultramaficassociation are olivine, orthopyroxene, clinopyroxene, andpyrope-almandine garnet, which at the transition zone are replacedby majorite garnet, orthorhombic wadsleyite [β-(Mg,Fe)2SiO4], andringwoodite with a spinel-type structure [γ-(Mg,Fe)2SiO4]. Accordingto experimental data, isochemical transformations in both olivinewadsleyiteand wadsleyite-ringwoodite take place at pressures of12–16 GPa and 18–22 GPa respectively (Fig. 2). At pressures of c.24 GPa and higher, wadsleyite and ringwoodite decompose to formferropericlase and a phase (Mg,Fe)SiO3 that has a perovskite-typestructure (‘MgSi-perovskite’) (e.g., Ringwood and Irifune, 1988;Chudinovskikh and Boehler, 2001). This mineral association is primaryindicator of lower-mantle conditions (Fig. 2). Under the samepressure–temperature conditions, majorite transforms into MgSiperovskitewith a possible admixture of stishovite and an Al-phase(e.g. Irifune et al., 1996). Under the same conditions, phase CaSiO3
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