The Cause and Consequence of Compositional Anomalies at the Core-mantle Boundary

Waveform modeling and travel time analyses of dense seismic observations reveal a rapidly-varying low-velocity basal layer at the base of the mantle extending from the south Atlantic ocean to the Indian ocean. This basal layer has steeply dipping edges, rapidly varying thicknesses (0-300 km) and geometries, and anomalously low shear wave velocities decreasing from -2% at 300 km above the core-mantle boundary to -9% to -12% at the core-mantle boundary. The maximum P velocity decrease associated with the seismic anomaly is -3%. The structural features and velocity characteristics of the basal layer unambiguously suggest that it is a compositional anomaly. The seismic characteristics associated with this basal layer can be best explained by partial melt driven by a compositional change produced early in the Earth's history. In contrast, the seismic structures beneath the central and western Pacific ocean are dramatically different. The central Pacific exhibits a shear velocity gradient of about -3% over the bottom 300 km of the mantle; and the western Pacific region is characterized by many ultra-low velocity zones with P velocity reductions as large as -10% and vertical length-scales of tens kilometers and horizontal length-scales from tens kilometers to hundreds kilometers. I suggest that the seismic structure beneath the central Pacific may be reasonably explained by pure thermal effects within a thermal boundary layer and the ultra-low velocity zones in the western Pacific may reflect the existence of similar chemical heterogeneities in much smaller length scales and with larger degrees of partial melt or existence of small-scale convection at the base of the mantle. On the Earth's surface, the ocean floor in the south Atlantic ocean and the Indian ocean is noticed to have two distinct characteristics. Its ocean islands have unique isotope signatures and its major hotspots are relatively fixed to each other. The ocean islands in the south Atlantic ocean and the Indian ocean have high ⁸⁷Sr/⁸⁶Sr ratios and anomalously high ²⁰⁷Pb/²⁰⁴Pb and Pb²⁰⁸/²⁰⁴Pb ratios for a given ²⁰⁶Pb/²⁰⁴Pb ratio. These unique isotope signatures and its geographical distribution were termed the "DUPAL anomaly". Several studies also indicated that major hotspots in the south Atlantic ocean and the Indian ocean have little motions with respect to each other, while other hotspots in the Pacific ocean have large motions with respect to the "Atlantic-Indian" hotspot group and the mantle. The geographical distribution of the basal compositional anomaly strongly correlate with the geographic extent of the "DUPAL anomaly" and many major hotspots in the south Atlantic and Indian oceans. I suggest that the compositional anomaly near the core-mantle boundary could provide an explanation for the "DUPAL anomaly" observed at the Earth's surface, in terms of its geographical distribution, uniqueness and enrichments, and the fixity of the "Atlantic-Indian" hotspot group, if mantle flow or plume is able to entrain the distinct materials from this bottom compositional layer and carry them to the Earth's surface. In this scenario, this basal compositional anomaly represents an "ancient" enriched chemical anomaly and its presence serves as an anchor to mantle plumes that give rise to the relatively fixed hotspots above.