Phase transition and density of subducted MORB crust in the lower ...

250
skite contains less than 1 wt.% Al 2O 3 [23,32] and
transition temperature in such Al-poor Ca-perovskite
is too low. The depth of phase transition can be quite
variable in the upper to mid–lower mantle, due to the
variations in temperature of slabs and Al2O3 content
in Ca-perovskite (Fig. 7).
The tetragonal to cubic transition in Ca-perovskite
may also be ferroelastic-type. It is already known that
many distorted perovskites exhibit a ferroelastic behavior.
Acoustic velocity in these perovskites remarkably
drops across the structural transition to high
symmetry phases [46,47]. The subducted MORB
crust includes ~23% Al-bearing Ca-perovskite, and
phase transition in Ca-perovskite possibly causes
large seismic anomalies. Part of the lower mantle
seismic scatters/reflectors may be reconciled with
the phase transition in Al-bearing Ca-perovskite included
in subducted MORB crust. The most intense
M-phase anomaly observed at 1400–1600-km depth
[17,19] may be caused by the combined effects of
phase transitions in SiO2 phase and Ca-perovskite.
Acknowledgments
We thank Y. Tatsumi and S. Kaneshima for helpful
comments. The in-situ X-ray experiments were carried
out at SPring-8 (proposal no. 2004A3013-LD2np
and 2004B4013-LD2-np). Comments by anonymous
reviewers improved the manuscript.
Appendix A. Supplementary data
Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/
j.epsl.2005.06.035.
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