L. Wen
Department of Geosciences, State University of New York, Stony Brook, New York, USA
Recent developments of seismic techniques and extensive waveform modeling of dense high-quality seismic data allow us to discover and constrain seismic structures in many regions of the core-mantle boundary. In this presentation, I will present seismic evidence for a rapidly varying low-velocity seismic anomaly at the base of the mantle, extending from the south Atlantic ocean to the Indian ocean [Wen et al., 2001; Wen, 2001; Wen, 2002]. These seismic observations indicate that the low-velocity anomaly 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 (relative to PREM). The maximum P velocity decrease associated with the seismic anomaly is -3\%. More excitingly, this seismic anomaly geographically coincides with the geochemical DUPAL anomaly both in the south Atlantic ocean and in the Indian ocean. The structural features and velocity characteristics of the anomaly unambiguously suggest that it is a compositional anomaly. The seismic characteristics associated with this anomaly can be best explained by partial melt driven by a compositional change produced early in the Earth's history. In another word, the seismic anomaly represents a partial melting region defined by its distinct composition with a depressed solidus and/or an elevated temperature, and the strong shear velocity reductions inside the anomaly reflect different fractions of melt and different temperatures at different depths of a positive thermal gradient at the bottom of the mantle. The proposed origin of this anomaly, i.e., an enriched chemical anomaly produced early in the Earth's history, is also in line with the early development of enrichments of the DUPAL anomaly suggested by the isotope data [Hart, 1984; 1988]. This compositional anomaly could provide an explanation for the distinctive DUPAL geochemical anomaly in terms of its geographical distribution, uniqueness, and enrichments. Placed in the global context of the core-mantle boundary regions, this seismic anomaly is dramatically different from the seismic structures beneath the central Pacific and the ultra-low velocity zones elsewhere. The bottom 300 km of the mantle beneath the central Pacific is inferred to have a negative shear velocity gradient of -3\% [Wen, 2002], and the ultra-low velocity zones have vertical scales of tens of kilometers and P velocity reductions of about -10\%. The ultra-low velocity zones could represent the existence of similar compositional anomalies in different length-scales and with different degrees of partial melt. On the other hand, the negative shear velocity gradient of -3\% beneath the central Pacific may reasonably be explained by pure thermal effects of a bottom thermal boundary layer of the mantle. Wen, L., Silver, P., James, D. and Kuehnel, R., Seismic evidence for a thermo-chemical boundary layer at the base of the Earth's mantle, Earth Planet. Sci. Lett., 189, 141-153, 2001. Wen, L., Seismic evidence for a rapidly-varying compositional anomaly at the base of the Earth's mantle beneath the Indian ocean, Earth Planet. Sci. Lett., 194, 83-95, 2001. Wen, L., An SH hybrid method and shear velocity structures in the lowermost mantle beneath the central Pacific and South Atlantic oceans, J. Geophys. Res., 107, B3, 10.1029/2001JB000499 29 March 2002, 2002. Hart, S.R., A large-scale isotope anomaly in the Southern Hemisphere mantle, Nature, 309, 753-757, 1984. Hart, S.R., Heterogeneous mantle domains: signatures, genesis and mixing chronologies, Earth Planet. Sci. Lett., 90, 273-296, 1988.