Mantle bulk sound velocity

Figure 5 Contours of r.m.s misfit (%) to seismological reference model akl35 of density (red) and bulk sound velocity (green) for candidate lower-mantle compositions, parametrized in terms of Mg/(Mg -f Fe) (= Xmj) and Si/(Mg -f Fe)(= Xpv), over the entirety of the lower mantle. Shaded region at Xpv > 1 indicates free silica. Triangle denotes pyrolite. Plus signs denote minima of r.m.s. misfit. Root of lower-mantle adiabat is 2,000 K at 660 km depth.

Figure 8 Depthwise best-fit compositions to seismological reference model akl35 for density alone (red), bulk sound velocity alone (green), and both density and bulk sound velocity jointly (blue), with compositions parametrized in terms of Mg/(Mg - - Fe) (XMg) and Si/(Mg - - Fe) (Xpv), in 10 km depth slices through the lower mantle. Shaded region at Xp, > 1 indicates free silica. Dotted lines (at Xp, = 0.67 and Xjy[g = 0.89) denote pyrolite. Root of lower-...

Figure 10 Temperature dependence of the bulk sound velocity versus depth in the lower mantle, for candidate bulk compositions for the basaltic layer (red), the underlying harzburgite layer (green), and the basal Iherzolite (blue). Note that temperature sensitivity falls with increasing depth.

The temperature dependence of the bulk sound velocity (8V/8T) for all of the compositions in the layered slab falls from below 0.2% per 100 K at the top of the lower mantle to below 0.1% per 100 K at the base of the lower mantle (Figure 10). As the magnitude of thermal anomalies associated with cold slab material will also fall with increasing depth as slabs thermally assimilate, this behavior suggests that it will be difficult to explain any large velocity anomalies at depth in terms of temperature alone. Indeed, even if thermal anomalies —100 K were to somehow survive down to the base of... 

Densities and bulk sound velocities in the lower mantle are also consistent with the high-pressure mineralogy of a bulk composition approximating an upper-mantle peridotite, such as pyrolite. Seismic constraints provide no support for iron enrichment of the lower mantle relative to such an upper mantle. Silica enrichment of the lower... 

Bina C. R. and Silver P. G. (1990) Constraints on lower mantle composition and temperature from density and bulk sound velocity profiles. Geophys. Res. Lett. 17, 1153—1156. 

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