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Elasticity of single‐crystal NAL phase at high pressure: A potential source of the seismic anisotropy in the lower mantle
Author(s) -
Wu Ye,
Yang Jing,
Wu Xiang,
Song Maoshuang,
Yoshino Takashi,
Zhai Shuangmeng,
Qin Shan,
Huang Haijun,
Lin JungFu
Publication year - 2016
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1002/2016jb013136
Subject(s) - anisotropy , materials science , phase transition , phase (matter) , bulk modulus , isotropy , geology , mineralogy , condensed matter physics , chemistry , composite material , optics , physics , organic chemistry
The new hexagonal aluminous phase, named the NAL phase, is expected to be stable at depths of <1200 km in subducted slabs and believed to constitute 10~30 wt% of subducted mid‐ocean ridge basalt together with the CaFe 2 O 4 ‐type aluminous phase. Here elasticity of the single‐crystal NAL phase is investigated using Brillouin light scattering coupled with diamond anvil cells up to 20 GPa at room temperature. Analysis of the results shows that the substitution of iron lowers the shear modulus of the NAL phase by ~5% (~6 GPa) but does not significantly affect the adiabatic bulk modulus. The NAL phase exhibits high‐velocity anisotropies with AV P = 14.7% and AV S = 15.12% for the Fe‐bearing phase at ambient conditions. The high AV S of the NAL phase mainly results from the high anisotropy of the faster V S 1 (13.9~15.8%), while the slower V S 2 appears almost isotropic (0.1~2.8%) at ambient and high pressures. The AV P and AV S of the NAL phase decrease with increasing pressure but still have large values with AV P = 11.4% and AV S = 14.12% for the Fe‐bearing sample at 20.4 GPa. The extrapolated AV P and AV S of the Fe‐free and Fe‐bearing NAL phases at 40 GPa are larger than those of bridgmanite at the same pressure. Together with its spin transition of iron and structural transition to the CF phase, the presence of the NAL phase with high‐velocity anisotropies may contribute to the observed seismic anisotropy around subducted slabs in the uppermost lower mantle.