Possible origin of the ULVZs beneath central Pacific: a δlnVs/δlnVp = 2:1 velocity reduction by FeOOHx

Ultra-low velocity zones (ULVZs) at the base of Earth's lower mantle have been popularly attributed to partial melting of local constitutes of the lower mantle. Due to the highly differentiated influence of melts on shear and compressional waves, partial melting is believed to be an excellent candidate for the origin of the ULVZs with high velocity reduction ratio between shear and compressional waves, i.e. δlnVs/δlnVp = 3:1 (e.g. Williams and Garnero, 1996; Berryman, 2000). These ULVZs resulted from partial melting should be located at the hottest regions among the heterogeneous temperature distributions. Such hottest spots normally lie inside large low shear velocity provinces (LLSVPs). Recent studies indicate that the ULVZs beneath central Pacific are distinctly located at the margin of the LLSVPS (e.g. Zhao et al. 2017; Ma et al. 2019) and the velocity reduction ratios between shear and compressional waves for the best fit of seismic data through these ULVZs (i.e. 2:1) are much lower than most of other ULVZs. The Central Pacific ULVZs are often accompanied by a density elevation (e.g. Hutko et al. 2009). Our time-resolved in situ synchrotron XRD study indicates that hydrogen content (x) of pyrite-FeOOH_x phase may stabilize at a level of 0.7 under the lower mantle P/T conditions, and the collective PVT data of pyrite-FeOOH_x with such composition may contribute to seismic velocity reductions with δlnVs/δlnVp = 2:1 and a density elevation up to 24%. As the pyrite-FeOOH_x can be produced when water molecules in the mantle meet hot iron in the core, matching of this new mechanism of ULVZ initiation with the ULVZs of low δlnVs/δlnVp ratio and elevated density may provide insight to mantle dynamics and composition.