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Deep Earth Elastic Properties and Effective Rheology

Periodic Reporting for period 1 - DEEPER (Deep Earth Elastic Properties and Effective Rheology)

Reporting period: 2015-07-30 to 2017-07-29

The Earth core-mantle boundary, is marked with a patchwork of structures with an anomalous seismic signature. The speed of seismic shear waves slow down substantially as they travel through these structures, also known as UltraLow Velocity Zones (ULVZs). Despite a number of observational and laboratory simulations of the ULVZs, two questions still remain unanswered: (a) is the seismic signature of the ULVZ caused by Fe-rich solids or partial melts? and (b) what is the internal structure of the ULVZs. The work in this project was aimed at addressing these two crucial questions using an array of new theoretical and numerical models in computational multiphase flow. The outcome of this project will be beneficial to a large group of Earth scientists studying the thermal and chemical evolution of the Earth, high performance computational scientists, and those studying fluid mechanics. The overall objectives of this project was to understand the relative influence of melting vs solid state composition on the physical properties of the ULVZs and studying the effect of convective motion in the lower mantle on the internal structure of the ULVZs.
In order to meet the objectives of this project, we developed a high resolution finite elements model of a partially molten, irregular shaped ULVZ patch. We also carried out a series of numerical experiments studying the effect of shearing from mantle motion in redistribution of the melt within the ULVZs. Our numerical models were supplemented by analytical solutions and linear stability analyses. The results from our simulations indicate that the distribution of melt within the ULVZs is strongly dependent on two factors: (a) the density contrast between the melt and the solid and (b) shearing of the ULVZs as they are swept by flow in the surrounding mantle. We also found that the boundary conditions used in the numerical simulations deeply influence the predicted structure. There has been only one previous study in this area, and this study wasn't able to address the boundary condition issues due to their technical limitation.

The duration of this project was reduced as the Fellow received a tenure-track academic position at a burgeoning new research institute in India and had to leave early. Despite the early termination, the results from this project were presented in two major international conferences, the Annual fall meeting of the American Geophysical Union in 2015 and the Annual European Geosciences Union conference in Spring 2016. In addition, these results were also disseminated as posters and talks at a workshop held at the University College of London and an international workshop on melting in the mantle hosted by the Isaac Newton Insittute at Cambridge University, UK.
The main progress of this project is the development of 3D numerical model of two-phase flow in the ULVZs. This is a significant achievement beyond the current results. The only two-phase models of the ULVZ are one dimensional. There are a few published 2D models of flow in the ULVZ, but these models are only for a single phase.