Influence of the iron spin transition in ferropericlase on global tomography

Global tomography models routinely image seismic velocities at depth in the mantle. By comparing model velocities with predictions for different mineralogical models one can attempt to constrain the mineralogy of the mantle. This way, it has been shown that the post-perovskite phase is a possible explanation for the negative correlation between shear-wave velocity (Vs) and bulk sound velocity variations (Vc) in the deep mantle in model SP12RTS. However, the onset of the negative correlation occurs at shallower depths in SP12RTS, which is not reproduced by geodynamic model predictions.

Here, we hypothesise that the iron spin transition in ferropericlase can resolve this mid mantle discrepancy. Although the transition from a high spin to low spin state does not produce a sharp seismic discontinuity in the lower mantle, mineral physics studies indicate that it has nonetheless a noticeable effect on seismic velocities. Particularly, Vc softens, while Vs is relatively unchanged, resulting in the required negative correlation. However, as ferropericlase only forms ~20 % of the Earth's mantle, the question is whether the spin transition effect can be observed in global tomography and thus explain the features of SP12RTS at mid mantle depths.

To investigate this, we have generated synthetic seismic models of the Earth's mantle based on geodynamic simulations of mantle flow combined with mineral physics lookup tables. We include the mixed spin state of ferropericlase in these tables from first principles thermodynamic calculations, while also incorporating the post-perovskite phase in the deep mantle. We generate synthetic tomography models with the resolution operator of SP12RTS, which enables direct comparisons between the tomography model and geodynamic predictions. Our analysis focuses on the relationships between the different velocities in the models as well as the radial correlation of the models themselves. Preliminary results indicate that the effect of the spin transition is visible in global tomography and leads to an improved fit to tomography.