title: The effect of Fe and Al on the elasticity of akimotoite
Creators: name: Siersch, Nicki
Publisher: name: University of Bayreuth
Year: 2019
Subjects: subject: Akimotoite
subject: high-pressure
subject: transition zone
subject: elasticity
subject: 550

description: The Earth’s transition zone between 410 and 660 km depth is a region of raised seismic wave velocity gradients separating the upper and lower mantle and is also marked by a number of seismic discontinuities. Mineralogical models for peridotitic and mid-ocean ridge basalt (MORB) lithologies fail to reproduce the high seismic velocities indicated by reference models for the base of the transition zone (550-660 km), due to the prevalence of majoritic garnet in these lithologies, which propagates seismic waves relatively slowly. A lower average mantle temperature at the base of the transition zone due to an accumulation of subducted lithosphere would be one way to explain the higher velocities. However, a further possibility is that the mantle at this depth contains melt depleted harzburgite material where majoritic garnet is replaced by the mineral akimotoite. Akimotoite is an MgSiO3 polymorph stable between 20 and 25 GPa and 900 to 2200 K and if present it might be responsible for raising the seismic wave velocities. Additional seismic discontinuities identified near the base of the transition zone might also be attributed to phase transitions involving akimotoite, as could some seismic anisotropy observations. The objectives of this study were to determine the elastic properties, anisotropic behavior and structural evolution of MgSiO3 akimotoite up to transition zone pressures. Furthermore, the effects of Al and Fe on the elasticity of akimotoite were investigated at high pressures and high temperatures. This enabled seismic wave velocities of relevant mantle lithologies to be accurately determined at these conditions and, through a comparison with seismic reference models, constraints were placed on the composition of the lower transition zone. In the first part of the study, large single-crystals of MgSiO3 akimotoite were synthesized and used to investigate the full elastic tensor by means of single-crystal Brillouin spectroscopy up to transition zone pressures. In combination with single-crystal X-ray diffraction, the bulk and axial compressibility of the same crystals were studied. Results show that the wave velocities of MgSiO3 akimotoite are faster than those of majoritic garnet. Furthermore, akimotoite remains a highly anisotropic mineral at the base of the transition zone even though the anisotropy decreases with pressure. Akimotoite axial compressibility is highly anisotropic because oxygens of the face-sharing octahedra lie along the a-axis and form a very stiff unit along that direction. In the second part of this study, the crystal chemistry of the MgSiO3 akimotoite – Al2O3 corundum solid solution was investigated through X-ray structure determinations on eight single-crystals of different composition. The unit-cell volume change across the akimotoite-corundum solid solution is near linear but the lattice parameters deviate strongly from ideality. Furthermore, the influence of Al on the elasticity of akimotoite was studied on two polycrystalline Al-bearing akimotoite samples up to 25 GPa and 1100 K using ultrasonic interferometry combined with synchrotron X-ray diffraction and radiography in a large volume press. A decrease in wave velocities was observed with increasing Al content. This is an unexpected result since wave velocities in corundum are much faster than in akimotoite. This strongly non-ideal behavior means that corundum properties cannot be employed as those of the akimotoite Al2O3 end-member, as previously assumed. By the same argument it is unlikely that akimotoite is an effective MgSiO3 end-member of corundum. The effect of Fe2+ substitution on the elastic properties of a polycrystalline akimotoite sample were investigated in the third part of this study using ultrasonic interferometry and synchrotron X-ray diffraction up to 26 GPa and 800 K. The incorporation of Fe was also found to reduce the compressional and shear wave velocities. This effect is further increased when substituting both Fe and Al into the akimotoite structure, which was investigated up to 23 GPa. The coupled substitution of Fe3+ and Al into MgSiO3 akimotoite is shown to have a major effect on the structure, most likely as a result of Al and Fe3+ being randomly distributed across both cation sites. Lastly, the obtained elastic properties of MgSiO3 akimotoite and those of Fe- and Al-bearing solid solutions were used to calculate the elastic properties of fictive FeSiO3 and Al2O3 akimotoite end-members. These data were used both in a thermodynamic model to determine the phase relations of akimotoite in a harzburgite assemblage between 20.5 and 23 GPa and to calculate the seismic velocity for this assemblage along a mantle adiabat. Above 22 GPa, the wave velocities for harzburgite, in contrast to peridotite and MORB lithologies, are found to be in excellent agreement with seismic reference models. The observation of multiple discontinuities in the proximity of the 660 km discontinuity is consistent with the sharp transformation of garnet to akimotoite determined to occur between 600 and 625 km. These observations provide strong evidence for the accumulation of harzburgitic material at the base of the transition zone.