Seismology mantle composition

Most seismological constraints on mantle composition are derived by comparison of values of seismic wave velocities inferred for particular regions within the Earth to the values measured in the laboratory for particular minerals or mineral assemblages, with such comparisons being made under comparable regimes of pressure (P) and temperature (T). The primary parameters of interest, then, are the compressional (or P-) wave velocities (Vp) and the shear (or S-) wave velocities (Ej). These wave velocities are simply related to the density (p) and to the two isotropic elastic moduli, the adiabatic bulk modulus (Ks)... 

Seismological Constraints upon Mantle Composition (MgpgFeQ ])jSi04 equilibria... 

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.

Seismological Constraints upon Mantle Composition Joint r.m.s.% misfit to p and... 

Aside from the core-mantle boundary region, a pyrolite lower-mantle composition appears to be consistent with seismological constraints. Silica enrichment of the lower mantle can be accommodated if the lower mantle is hotter than expected for a simple adiabat rooted at the 660 km y— pv + mw transition (Figure 9). Because any chemical boundary layer between the upper and lower mantle would be accompanied by a corresponding thermal boundary layer, such a model... 

Jackson I. and Rigden S. M. (1998) Composition and temperature of the Earth s mantle seismological models interpreted through experimental studies of Earth materials. In The Earth s Mantle Composition, Structure and Evolution (ed. I. Jackson). Cambridge University Press, Cambridge, pp. 405-460. 

Structure and dynamics of the lowermost mantle. This region includes the D layer, which is characterized by major chemical and thermal variations. It is likely of fundamental importance to the chemical evolution of the mantle and may function as a (temporary) resting place for subducted slabs. It is also expected to influence the stability of mantle plumes (Davaille et al., 2002 Jellinek and Manga, 2002), the entrainment and residence times of chemical heterogeneity (Olson and Kincaid, 1991 Schott et al., 2002, and the thermal, chemical, and seismological characteristics compositional variations (Kellogg et al., 1999 Tackley, 2002). 

Seismological evidence has led to the realization that the Earth is a layered body, with the basic subdivisions into crust, mantle, and core being further refined to give an upper and lower mantle separated by a transition zone, and a liquid outer core and solid inner core. The different layers show fairly sharp contrasts in density and elastic properties, leading to speculation that changes in crystal structure (and possibly in electronic structure) as well as in overall composition are responsible for the layer-... 

Direct sampling of mantle rocks and minerals is limited to tectonic slices emplaced at the surface (see Chapter 2.04), smaller xenoliths transported upwards by magmatic processes (see Chapter 2.05), and still smaller inclusions in such far-traveled namral sample chambers as diamonds (see Chapter 2.05). Because of such limited direct access to mantle materials, knowledge of mantle structure, composition, and processes must be augmented by geophysical remote sensing. What can various seismological observations tell us about the major-element composition of the upper mantle How can they constrain possible differences in chemical composition between the upper... 

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-...

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