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Insights into the mineralogy of the lower mantle using comparisons between global seismic tomography and geodynamic models [Rescheduled to online] - Dr. Paula Koelemeijer, Royal Holloway
2020-05-27 11:00 - 12:00 UTC | ONLINE - Details to be sent by email
*This seminar will be held online. Details will be sent to the Earth Sciences and Bullard mailing lists. If you are not in Earth Sciences but would like to attend, please contact the talk organisers.*

Global tomography models routinely image seismic velocities at depth in the mantle. However, interpretations of these seismic velocities are not unique due to competing effects of temperature, composition and phase transitions. One way to still obtain insights into the structure of the mantle is by comparing seismic tomography models to predictions based on geodynamic models of mantle convection. These models serve to provide a reasonable temperature distribution, while the effects of different compositions can be investigated through hypothesis testing of particular scenarios. One important aspect to take into account in such comparisons is the limited resolution of global tomography models.

In my tomographic-geodynamic comparisons I focus on the mid and the lowermost mantle, where tomographic models consistently find a negative correlation between shear-wave velocity (Vs) and bulk sound velocity (Vc) variations. As temperature effects are expected to have a similar effect on these velocities, this observation has long served as argument for large-scale compositional variations. I will show instead that the phase transition from bridgmanite to post-perovskite has a stronger influence on the Vs-Vc correlations and can serve as a possible explanation in the deep mantle. However, the onset of the negative correlation occurs at shallower depths (~1800 km), which is not reproduced by either thermal or thermochemical models with post-perovskite. I hypothesise that the iron spin transition in ferropericlase, which has a strong effect on Vc, can resolve this mid mantle discrepancy. Although ferropericlase only forms ~20 % of the Earth’s mantle, I will use new first principles calculations for the mixed spin state to show that this spin transition effect can be observed in global tomography and leads to an improved fit to seismic tomography.
