The post-stishovite phase transition in hydrous alumina-bearing SiO 2 in the lower mantle of the earth

Dmitry L. Lakshtanov, Stanislav V. Sinogeikin, Konstantin D. Litasov, Vitali B. Prakapenka, Holger Hellwig, Jingyun Wang, Carmen Sanches-Valle, Jean Philippe Perrillat, Bin Chen, Maddury Somayazulu, Jie Li, Eiji Ohtani, Jay D. Bass

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Abstract

Silica is the most abundant oxide component in the Earth mantle by weight, and stishovite, the rutile-structured (P42/mnm) highpressure phase with silica in six coordination by oxygen, is one of the main constituents of the basaltic layer of subducting slabs. It may also be present as a free phase in the lower mantle and at the core-mantle boundary. Pure stishovite undergoes a displacive phase transition to the CaCl2 structure (Pnnm) at ≈55 GPa. Theory suggests that this transition is associated with softening of the shear modulus that could provide a significant seismic signature, but none has ever been observed in the Earth. However, stishovite in natural rocks is expected to contain up to 5 wt % Al2O3 and possibly water. Here we report the acoustic velocities, densities, and Raman frequencies of aluminum- and hydrogen-bearing stishovite with a composition close to that expected in the Earth mantle at pressures up to 43.8(3) GPa [where (3) indicates an uncertainty of 0.3 GPa]. The post-stishovite phase transition occurs at 24.3(5) GPa (at 298 K), far lower than for pure silica at 50-60 GPa. Our results suggest that the rutile-CaCl2 transition in natural stishovite (with 5 wt % Al2O3) should occur at ≈30 GPa or ≈1,000-km depth at mantle temperatures. The major changes in elastic properties across this transition could make it visible in seismic profiles and may be responsible for seismic reflectors observed at 1,000- to 1,400-km depth.

Original languageEnglish
Pages (from-to)13588-13590
Number of pages3
JournalProceedings of the National Academy of Sciences of the United States of America
Volume104
Issue number34
DOIs
Publication statusPublished - 2007 Aug 21

Keywords

  • Brillouin scattering
  • High pressure
  • Hydrogen
  • Phase transition
  • Silica

ASJC Scopus subject areas

  • General

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    Lakshtanov, D. L., Sinogeikin, S. V., Litasov, K. D., Prakapenka, V. B., Hellwig, H., Wang, J., Sanches-Valle, C., Perrillat, J. P., Chen, B., Somayazulu, M., Li, J., Ohtani, E., & Bass, J. D. (2007). The post-stishovite phase transition in hydrous alumina-bearing SiO 2 in the lower mantle of the earth. Proceedings of the National Academy of Sciences of the United States of America, 104(34), 13588-13590. https://doi.org/10.1073/pnas.0706113104