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Q. Williams, J. Revenaugh, E. Garnero*
The statistical correlation between the locations of hot spots at the surface of Earth and the distribution of ultra-low-velocity zones at the base of the mantle has about a 1 percent chance of arising randomly. This correlation is more significant than that between hot spots and negative velocity anomalies in tomographic models of deep mantle compressional and shear velocity. This correlation is consistent with the notion that many hot spots originate in a low-velocity, probably partially molten layer at the core-mantle boundary and undergo little lateral deflection on ascent.
The underlying control on the geographic distribution of hot spots, linear chains of volcanic edifices whose source appears to be fixed relative to surface plate motions, is uncertain. Hot spots tend to be distributed near long-wavelength geoid highs (1) and mid-ocean ridges (2), each of which may in turn be associated with slow seismic velocities in the lower mantle (3, 4). The upwellings that give rise to hot spots are widely thought to originate as instabilities near the core-mantle boundary (CMB) (5, 6), as this region likely represents a major thermal boundary layer. Geophysical observations that support hot spots originating near the CMB have, however, been notably lacking (7), although possible geochemical evidence for such a provenance exists (8). Here we examine whether hot spots are correlated with the presence of recently discovered 5- to 40-km-thick features at the base of Earth's mantle with compressional wave velocities depressed by as much as 10% from the overlying mantle (9-13). These ultra-low-velocity zones (ULVZs) are likely generated by the presence of partial melt at depth (10, 11); it is unclear whether this partial melt differs chemically from the overlying mantle through (for example) either iron enrichment or volatile enrichment (10).
Thus far, the Fresnel zones of seismic waves sample 44% of the CMB for the presence or absence of ULVZs and ULVZs have been observed to be present over 12% of the CMB (12, 13). The locations of the ULVZs are derived from diffracted compressional wave segments traveling along the mantle side of the CMB (9, 12, 14) and from reflected compressional waves that sample the upper boundary of this basal layer (1, 13) (Fig. 1). Where ULVZs have been...