Elasticity across the post-stishovite transition in subducted basalt

Zhang, Yanyao
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Regional seismic studies have found many small-scale scatterers with shear wave velocity (Vₛ) anomalies near subducting slabs in the lower mantle, indicating chemical anomalies at depth. Subducting slabs transport surface hydrated mid-ocean ridge basalt (MORB) into deep Earth, which is chemically and physically distinct from the surrounding mantle. Physical properties of MORB materials are thus important to understand observed seismic Vₛ anomalies. In this dissertation, physical properties of stishovite and CaCl₂-type post-stishovite, which are abundant MORB components, have been determined at high pressure in order to study geophysical consequences across the post-stishovite transition in MORB. Specifically, I measured sound velocities using Brillouin light scattering and impulsive stimulated light scattering, Raman shifts of optic modes using Raman spectroscopy, and atomistic properties using synchrotron single-crystal X-ray diffraction in diamond anvil cells at high pressure. Sound velocity results show that the elastic modulus C₁₂ of pure-endmember stishovite converges with C₁₁ at 55 GPa where the shear wave Vₛ₁ propagating along [110] direction becomes zero and aggregate shear wave Vₛ drops by ~26%. Microscopically, this abnormal elastic property can be correlated with a crossover of the apical and equatorial Si-O bond lengths that results in the occurrence of the symmetry-breaking spontaneous strain in the post-stishovite phase. Furthermore, the Al and H incorporation into stishovite lattice structure can significantly reduce transition pressure and slightly enhance Vₛ reduction. For example, stishovite with 1.3-2.1 mol % Al undergoes the ferroelastic transition at ~16-21 GPa where Vₛ drops by ~29%. This Al,H-dependent post-stishovite transition in subducted MORB materials has been used to explain depth-dependent small-scale seismic Vₛ anomalies beneath the Tonga subduction region. The coupled H and Al substitution, together with literature element partitioning data, indicates that stishovite with 1.3 mol% Al in the upper part of the lower mantle could accommodate ~0.3 wt% H₂O.