Sound velocities of aluminous MgSiO₃ perovskite

J.M. Jackson^1*, J. D. Bass¹ and J. Zhang^2**

¹Department of Geology, University of Illinois, Urbana, IL, U.S.A.
²Department of Geosciences, SUNY at Stony Brook, NY, U.S.A.

* jmjackso@uiuc.edu

** current address: Los Alamos National Laboratory, Los Alamos, NM, U.S.A.

Accurate laboratory-derived equations of state for aluminum-bearing silicate perovskite are necessary for constraining the chemical and physical state of the deep Earth. Based on numerous theoretical and experimental studies, aluminous (Mg,Fe)SiO₃ perovskite is thought to represent the dominant phase in Earth's lower mantle. To date, the effect of Al³⁺ in MgSiO₃ perovskite on the sound velocities and shear modulus of elasticity is unknown. Furthermore, recent compression measurements using X-ray diffraction techniques do not yield consistent results for the effect of aluminum on the isothermal bulk modulus of silicate perovskite, therefore making it difficult to constrain models of Earth's lower mantle. We present the first Brillouin scattering results on MgSiO₃ perovskite containing 5 mol% Al₂O₃ . The polycrystalline sample used in this study was synthesized from a glass at about 25 GPa and 1873 K in a Kawai-type multi-anvil apparatus. Notably, velocities from Brillouin scattering experiments are insensitive to porosity in the polycrystalline samples, and the bulk and shear moduli are obtained directly (independent of any equation of state). Our results show that small amounts of aluminum present in MgSiO₃ perovskite decrease the compressional VP and shear VS wave velocities and the bulk (K) and shear (m) moduli, as compared to Al-free MgSiO₃ perovskite. However, the effect of aluminum on the bulk modulus is not as dramatic as previously claimed [1-4]. What is perhaps more important is that we find an ~8% decrease in the shear modulus of Al-Pv as compared with MgSiO₃ perovskite [e.g., 5].

[1] Zhang J. and Weidner D.J., Science 284, 782 (1999).
[2] Daniel I., Cardon H., Fiquet G., Guyot F. and Mezouar M., Geophys. Res. Let. 28, 3789 (2001).
[3] Kubo, A., Yagi, T., Ono, S., and Akaogi, M., Proc. Jap. Acad., 76, 103-107 (2000).
[4] Yagi, T., Okabe, K. and Nishiyama, N., submitted (20003).
[5] Yeganeh-Haeri, A., Phys. Earth Planet. Int., 87, 111-121 (1994).

Supported by NSF 0003383 and the COMPRES Elasticity Grand Challenge