Precise determination of post-stishovite phase transition boundary and implications for seismic heterogeneities in the mid-lower mantle

Details Precise determination of post-stishovite phase transition boundary and implications for seismic heterogeneities in the mid-lower mantle Precise determination of post-stishovite phase transition boundary and implications for seismic heterogeneities in the mid-lower mantle

Nov 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010pepi..183..104n&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 183, Issue 1-2, p. 104-109.

Physics

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Scientific paper

The phase transition boundary between stishovite and CaCl 2 -type structure in pure SiO 2 was investigated at 45-75 GPa and 300-2490 K based on the in situ X-ray diffraction measurements in a laser-heated diamond-anvil cell (LHDAC). Results show that the boundary has a positive d P /d T slope of 11.1 MPa/K and the transition occurs about 70 GPa at 2200 K along the typical mantle geotherm. This corresponds to a depth of 1700 km, somewhat shallower than the previous estimate. Seismic heterogeneities have been found in a wide depth range from 800 to 1850 km in the upper to middle layers of the lower mantle. The post-stishovite phase transition is known to be ferroelastic-type, which causes a considerable reduction in shear modulus, leading to a large slow shear velocity anomaly. The almost pure SiO 2 phase included in subducted crustal materials may be therefore responsible for the relatively deep mid-lower mantle seismic heterogeneities. The shallow lower mantle anomalies are possibly caused by the similar phase transition in SiO 2 phase containing Al 2 O 3 and H 2 O or by the phase transition in Al-bearing CaSiO 3 perovskite from cubic to tetragonal structure.

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