Single-crystal elasticity of iron-bearing majorite to 26 GPa: Implications for seismic velocity structure of the mantle transition zone

Motohiko Murakami, Stanislav V. Sinogeikin, Konstantin Litasov, Eiji Ohtani, Jay D. Bass

doi:10.1016/j.epsl.2008.07.045

Abstract Single-crystal elastic moduli of synthetic iron-bearing majoritic garnet were measured using Brillouin spectroscopy up to a pressure of 26 GPa. We found that the iron-bearing majoritic garnet (Mg 0.79 Fe 0.08 Al 0.30 Si 0.84 O 3 ) remains elastically isotropic throughout the pressure range of this study. The shear and adiabatic bulk moduli and their pressure derivatives at zero pressure were determined to be µ 0 = 88.7(7) GPa, K S0 = 166(2) GPa, (∂ µ / ∂ P ) 0 = 1.28(8), (∂ K / ∂ P ) 0 = 4.2(2). These values are almost indistinguishable, within the experimental uncertainties, from those obtained in previous studies of pure MgSiO 3 majorite and other solid solutions of the majorite–pyrope join. Our direct acoustic velocity measurements of iron-bearing majorite indicate that the incorporation of iron into majoritic garnet does not drastically affect its elastic moduli and their pressure derivatives. The measured pressure derivatives of iron-bearing majorite are about 3.6 times lower than required to account for the high seismic velocity gradients observed in the mantle transition zone.

Single-crystal elasticity of iron-bearing majorite to 26 GPa: Implications for seismic velocity structure of the mantle transition zone

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