TY - JOUR
T1 - The post-stishovite phase transition in hydrous alumina-bearing SiO 2 in the lower mantle of the earth
AU - Lakshtanov, Dmitry L.
AU - Sinogeikin, Stanislav V.
AU - Litasov, Konstantin D.
AU - Prakapenka, Vitali B.
AU - Hellwig, Holger
AU - Wang, Jingyun
AU - Sanches-Valle, Carmen
AU - Perrillat, Jean Philippe
AU - Chen, Bin
AU - Somayazulu, Maddury
AU - Li, Jie
AU - Ohtani, Eiji
AU - Bass, Jay D.
PY - 2007/8/21
Y1 - 2007/8/21
N2 - 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.
AB - 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.
KW - Brillouin scattering
KW - High pressure
KW - Hydrogen
KW - Phase transition
KW - Silica
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U2 - 10.1073/pnas.0706113104
DO - 10.1073/pnas.0706113104
M3 - Article
AN - SCOPUS:35148866979
SN - 0027-8424
VL - 104
SP - 13588
EP - 13590
JO - Proceedings of the National Academy of Sciences of the United States of America
JF - Proceedings of the National Academy of Sciences of the United States of America
IS - 34
ER -