Insights into the presence of post-perovskite in Earth's lowermost mantle from tomographic-geodynamic model comparisons
Abstract
Lower mantle tomography models consistently observe an increase in the ratio of shear-wave velocity (Vs) to compressional-wave velocity (Vp) variations in the lowermost mantle, accompanied by a significant negative correlation between Vs and bulk-sound velocity (Vc) variations. These seismic characteristics, also observed in the recent SP12RTS model, have traditionally been interpreted as indicative of large-scale chemical variations, though more recently the lower mantle post-perovskite (pPv) phase has also been invoked as a possible explanation. As geodynamical calculations of isochemical and thermochemical flow predict a fundamentally different style of mantle convection, interface topographies and core heat flow, we seek to answer the following questions: Are a high Vs/Vp ratio and a negative Vs-Vc correlation indicative of chemical variations? How much of the signal can be attributed to pPv? Can we distinguish between isochemical and thermochemical models using global tomographic models? To answer these questions, we compare first-order features of SP12RTS to synthetic tomographic images derived from whole mantle convection models. These models are converted to seismic velocities using constraints from mineral physics, reparametrised and convolved with the tomographic resolution operator. In contrast to previous studies, where only the Vs structures have been compared, we use both the Vs and Vp resolution operator of SP12RTS to allow direct comparisons of the resulting velocity ratios and correlations. We include geodynamic models with and without pPv and/or chemical variations to investigate the origin of the high Vs/Vp ratio and Vs-Vc anti-correlation. Although the tomographic filtering significantly affects the synthetic tomography images, we demonstrate that the patterns observed in the ratios and correlations of seismic velocities are robust features. Our study suggests that the seismic characteristics of SP12RTS require the presence of post-perovskite, both outside and inside the LLSVPs. However, these characteristics cannot be used to discriminate between isochemical and thermochemical models of mantle convection.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMDI52A..06K
- Keywords:
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- 1025 Composition of the mantle;
- GEOCHEMISTRYDE: 3924 High-pressure behavior;
- MINERAL PHYSICSDE: 7208 Mantle;
- SEISMOLOGY