Chemical composition of the deep mantle - investigation methods and challenges. Spin transitions, phase changes.
Following on from lecture 3, we have presented the various stages in the study of the Pv → pPv transformation in magnetic perovskite from its discovery in 2004 to the most recent results. Current controversies center around the effects of inhomogeneous composition (addition of aluminum and iron) on the Clapeyron slope of the transition (and therefore its position in the mantle or core), and on the thickness of this transition. The latest work in materials physics considers more realistic mineralogical compositions, such as the pyrolite model, or that of mid-ocean ripple basalts (MORB). It would seem that for this transition to be observed deep in the mantle, a composition close to that of subduction zones (MORB and harzburgite) is required, but there are still major uncertainties regarding experimental results.
In the second part of this lesson, we addressed the question of the observation of seismic anisotropy at the base of the mantle and its interpretation. After a brief introduction to the notion of seismic anisotropy and its interpretation (intrinsic and extrinsic anisotropy; radial and azimuthal anisotropy), we showed some examples of observations in the upper mantle, where this phenomenon can be easily understood within the framework of the dynamics associated with plate tectonics. We introduced the notion of volume wave birefringence, and its observation both in the upper mantle and in the Dʺ zone at the base of the mantle, where the radial anisotropy component is mainly detected using birefringence measurements of S-waves diffracted on the CMB. The large-scale distribution of radial anisotropy appears to correlate with that of seismic velocities. Interpretation is hampered by the fact that little is known about the dominant deformation regime at the corresponding pressures and temperatures. Knowing that post-perovskite appears to be much more anisotropic than perovskite, several groups are currently engaged in exercises to calculate seismic anisotropy as a function of the crystal structure of the main components of the Dʺ zone, the associated slip systems and the deformation field that results from mantle dynamics.
