The post-stishovite phase transition in hydrous alumina-bearing SiO2 in the lower mantle of the earth.
Silica is the most abundant oxide component in the Earth mantle by weight, and stishovite, the rutile-structured (P4(2)/mnm) high-pressure 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 CaCl(2) structure (Pnnm) at approximately 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 % Al(2)O(3) 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-CaCl(2) transition in natural stishovite (with 5 wt % Al(2)O(3)) should occur at approximately 30 GPa or approximately 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.
Department of Geology, University of Illinois at Urbana-Champaign, 1301 West Green Street, Urbana, IL 61801, USA. lakshtan@uiuc.edu
Dmitry L
DL
Lakshtanov
Stanislav V
SV
Sinogeikin
Konstantin D
KD
Litasov
Vitali B
VB
Prakapenka
Holger
H
Hellwig
Jingyun
J
Wang
Carmen
C
Sanches-Valle
Jean-Philippe
JP
Perrillat
Bin
B
Chen
Maddury
M
Somayazulu
Jie
J
Li
Eiji
E
Ohtani
Jay D
JD
Bass