The Cycling of Subducted Oceanic Crust in the Earth’s Deep Mantle

The oceanic crust is created at mid-ocean ridges and returned to the Earth’s deep mantle at subduction zones. A clear picture on the dynamics and distribution of the subducted oceanic crust in the deep mantle is crucial for understanding the nature of compositional heterogeneities and the chemical evolution of the deep Earth. This chapter reviews observational constraints, mineral physics experiments, and geodynamic studies on the distribution, physical properties, and dynamics of the subducted oceanic crust in the deep mantle. It has been found that the subducted oceanic crust may exhibit a variety of behaviors when interacting with the surrounding mantle, depending on the volume, density, and viscosity of the subducted crustal materials and the surrounding mantle dynamics. The subducted oceanic crust may sink directly to the lowermost mantle, or pond at the base of the mantle transition zone with some part episodically sinking to the bottom of the mantle. The subducted oceanic crust in the lowermost mantle may either be directly carried up to shallower depths by entrainment, or accumulate on the core-mantle boundary with some part entrained into, and carried upward by, mantle plumes. The entrained oceanic crust may be directly carried to the base of the lithosphere, pond at a variety of mantle depths, or sink downward due to its negative buoyancy, and it is eventually mixed into the background mantle. The cycling of subducted oceanic crust throughout the deep mantle provides an understanding on the geochemical complexities in volcanic rocks and the seismic anomalies in the deep mantle.