3.1 a. Garnet symmetry in the Earth's transition zone (S. Heinemann, T.G. Sharp, F. Seifert and D.C. Rubie)
Garnets are important (>40 vol%) constituents of the Earth's mantle transition zone (depth 400-660 km) and therefore contribute significantly to its chemical and physical properties. According to chemical analyses from inclusions in diamonds and from high-pressure experiments on complex natural starting materials, they may be described essentially by the binary solid solution series pyrope (Mg3Al2Si3O12) - majorite (Mg4Si4O12), with majorite concentrations up to ca. 70 mol% in the lower part of the transition zone. A crystal chemical characteristic of these garnets is their content of Si on the octahedrally coordinated site. Whereas pyrope-rich garnets are cubic under all conditions, the majorite end-member is tetragonal (through octahedral Mg-Si ordering) at least after quench. Because elastic properties of garnets may be strongly affected by a cubic-tetragonal phase transition, we have clarified the symmetry of majorite-pyrope garnets under the conditions of the Earth's transition zone.
Garnets along the join Mg4Si4O12 (majorite end member) - Mg3Al2Si3O12 (pyrope) synthesized at 2000°C, 19 GPa are, after quench, tetragonal in the compositional range up to 20 mol% pyrope, but cubic at higher Al contents. Lattice constants atet and ctet in the tetragonal compositional range converge with increasing pyrope contents towards the lattice constant of the cubic garnets (see Annual Report 1994). The elastic strain and the intensity of the (222) reflection of x-ray powder patterns as a function of composition indicate a second-order phase transition near 20 mol% pyrope. Microtextural features (cf. Section 3.3e) may be used to further elucidate the formation of the tetragonal phase. From the wedge-like shape of pseudomerohedral twins and their formation from 90o twin-boundary corners, as well as from the absence of growth-induced dislocations at 90o corners, it is concluded that the Al-poor garnets of the join Mg4Si4O12- Mg3Al2Si3O12 are also cubic at synthesis conditions but invert by (Mg,Si) ordering on the octahedral sites into tetragonal phases of space group I41/a upon quench. This implies that the cubic-to-tetragonal phase transition in Mg4Si4O12 garnet occurs below 2000°C at 19 GPa and even lower in more aluminous compositions. A composition-dependent Landau model is consistent with a direct transformation from space group Ia-3d to I41/a. Comparison of the temperature (T)- composition (X) stability field of majorite-pyrope garnets with the chemistry of majorite-rich garnets expected to occur in the Earth's transition zone (Fig. 3.1-1) shows that the latter will be cubic under all conditions. Softening of elastic constants which commonly accompanies ferroelastic phase transitions may be expected to affect the elastic constants of the cubic garnet phase. Therefore, the proximity of the phase transition may affect the seismic velocities of cubic majorite in the deeper transition zone where majorite contents are highest.
Fig. 3.1-1: Isobaric temperature (T)-composition (X) section of the system pyrope (Mg3Al2Si3O12) - majorite (Mg4Si4O12) at 19 GPa showing the derived equilibrium boundary between cubic and tetragonal garnets in comparison with the stability field of garnet solid solutions at this pressure (taken from Gasparik, J.Geophys. Res. 97, 15181, 1992). It follows that tetragonal garnets can only be formed outside their stability field. Filled symbols are projected compositions of majoritic garnets synthesized by various authors from complex (peridotitic or pyrolitic) starting materials in the pressure range 9-25 GPa, indicating that garnets are cubic in the Earth's transition zone.