The state of mantle minerals

Abstract

The static pressure-volume, high-pressure X-ray, shock, and ultrasonic equation of state data are used to describe the seismic properties in terms of the mineralogy of the upper mantle, the spinel-zone, and the post-spinel or lower mantle. For the upper mantle, elastic properties have been measured for olivines, pyroxenes, and garnets. The effect of pressure, temperature and the Fe/Mg ratio on elasticity is less well known for pyroxene and garnet than for olivine. In the spinel zone, ~ 400- ~ 650 km depth, the major minerals include β- and γ-(Mg,Fe)_2SiO_4 and a complex garnet, largely composed of aluminum-silicate garnet and pyroxene components. The zero-pressure bulk moduli of these phases lie in the range 1.8–2.1 Mbar. Although the high-pressure X-ray measurements of the bulk moduli of the Fe-rich γ- or spinel phase (ringwoodite) are reported, the shear properties of the silicate spinels are unmeasured, as are all the properties of the distorted spinel, β-phase. The bulk modulus of the complex-garnet phase is known approximately from shock-wave data for pyroxene, but since standard specific volumes are unknown, reduction of these data to adiabats is uncertain. The major minerals of the lower, or post-spinel, mantle are thought to be (Mg,Fe)_2SiO_4 in the close-packed Sr_2PbO_4-structure, and the pyroxene and garnet component, initially in the close-packed ilmenite structure, and at greater depths probably transforming to the perovskite structure. Only shock-wave data and results inferred from the elasticity of the close-packed oxides are available in this regime. Except for garnet, the zero-pressure densities of the shock-induced high-pressure phases are not known; this difficulty, and the lack of data or a theory for the Grüneisen parameter make reduction of these data difficult.

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