Spin crossover in ferropericlase at high pressure: a source of mantle heterogeneity?
Abstract
The characterization of pressure- and temperature-induced transitions of mantle minerals, and their link with seismic discontinuities, is one of the most striking contributions provided by mineral physics for the understanding of Earth's interior. Emblematic in this sense is the series of phase transformations that occur in olivine, which ultimately define the main seismic discontinuities of the upper mantle. On the contrary, the spin transition in ferropericlase and Fe-bearing perovskite has not yet been clearly associated to any seismic observation. Recent inelastic x-ray scattering measurements on (Mg0.83Fe0.17)O ferropericlase at pressures across the spin transition show that only the shear moduli of the elastic tensor are affected by the changes in the spin state, not the longitudinal ones [Antonangeli et al., Science 331, 64 (2011)]. This explains the absence in deviation in the aggregate seismic velocities and, thus, the lack of signature of the spin transition in one-dimensional seismic profiles. However, the elastic shear anisotropy of high-spin and low-spin ferropericlase are profoundly different. The measured shear anisotropy of ferropericlase with a Fe content of 17 mol% is ∼70% at 70 GPa. In regions of highly textured grains, with significant lattice preferred orientations (corresponding for instance to deformation due to convection), such a large single-crystalline elastic anisotropy could results into a seismically detectable shear-wave anisotropy. Accordingly, iron spin transitions (depending upon temperature and iron concentration) should be considered together with mineralogical and chemical variations as possible sources of lateral heterogeneities in the lower mantle.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMMR23C..06A
- Keywords:
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- 3909 MINERAL PHYSICS / Elasticity and anelasticity;
- 3924 MINERAL PHYSICS / High-pressure behavior;
- 3954 MINERAL PHYSICS / X-ray;
- neutron;
- and electron spectroscopy and diffraction;
- 7208 SEISMOLOGY / Mantle