Abstract
The origin of the wave velocity anomalies at the core-mantle boundary (CMB) has been controversial. The primordial helium reservoir in the deep lower mantle remains elusive even with geochemical evidence for its existence. Here, we calculated the density and wave velocity of the FeO2-FeO2He system under the CMB conditions using first principles. The FeO2 and FeO2He of pyrite-type can exist stably under the CMB conditions without melting, and the incorporated helium increases the stability of the system. The electrical properties of FeO2 and FeO2He are not related to pressure. Doped helium reduces the density of the system but increases the elastic modulus. Our results suggest that FeO2 can be used as a viable material composition of ultra-low velocity zones (ULVZs), and FeO2He can explain the D'' seismic discontinuity instead of ULVZs. The primordial helium reservoir possibly formed by the accumulation of FeO2He, the only stable solid helium-bearing compound under the CMB conditions, may coincide with the location of the D'' layer.
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