[S-IT20] Deep Earth Sciences

[SIT20-P08] The dynamical and elastic properties of ULVZs based on ab initio high-P,T melts data

*Kan Kitano¹, Taku Tsuchiya¹ (1.GRC, Ehime University )

Keywords:ULVZ, Ab initio calculations of melts, Viscosity, P-wave velocity

Ultra-low velocity zones (ULVZs) have been detected at the core-mantle boundary (CMB) in the deep Earth as thin layers, spanning several tens of kilometers, where seismic wave velocities drop significantly (Garnero and Helmberger, 1996). One possible explanation for them is the presence of partial melts (Williams and Garnero, 1996). Understanding the origin of ULVZs is important not only for better understanding of the current CMB but also for insights into the long-term evolution of the core and mantle since Earth’s formation. These unusual zones are thought to have remained stable at the CMB for long periods, for silicate melts to stay mechanically stable in the lowermost mantle, they need to be rich in heavy elements like Fe. We performed ab initio calculations of densities and structures of several type of melts under the CMB condition and presented that their densities mainly depend on the Fe content and ULVZs could acquire enough gravitational stability when they have the Fe concentration comparable to those in lunar basalts (Kitano and Tsuchiya, 2024, JpGU). However, our knowledge about the physical properties of such Fe-rich melts at the CMB pressure and temperature are still quite limited. To explore the fundamental physical properties of such Fe-rich melts, such as viscosity and elasticity, we further performed ab initio molecular dynamics (MD) simulations based on the density functional theory.
Viscosity is a key property in mantle dynamics, influencing the shape and extent of entrainment of ULVZs (Namiki, 2003). On the other hand, the P-wave velocity of melts is an essential quantity for evaluating the melt fraction and partial melt texture of the regions where ULVZs are observed (Takei, 2002). Since the specific chemical composition of ULVZs are still largely unclear, in addition to representative lithologies actually found on the Earth and Moon, MORB, lunar basalts, and carbonatites, the pyrolitic composition is adopted as the model compositions of ULVZs, and their viscosities and elasticities are calculated. Comparing the results of these multiple model compositions, the influences of the chemical composition on the physical properties of melts under the CMB condition (135.8 GPa and 4000 K) are investigated. The obtained viscosity of pyrolitic melt is consistent with the one reported in previous study of peridotitic melt (Huang et al., 2024). Meanwhile, it of MORB is lower than the one reported in Bajgain et al. (2022) for basaltic melt. This difference is speculated to originate in the difference in the composition containing Fe (this study) and without Fe (Bajgain et al., 2022). The viscosities of lunar basalt and carbonatite melts, calculated for the first time, are found to be slightly and largely smaller than it of MORB, respectively. Additionally, this study might be the first calculations of the P-wave velocities for all the melts. The dynamical and elastic properties of ULVZs are modeled using calculated properties.