Upper mantle structure

The Emici-Roccamonfina zone has a crustal thickness of about 30 km. The uppermost mantle is characterised by a thin layer of material with relatively low S-wave velocity (Vs = 3.95 km/sec), which passes into a thick lid that has higher S-wave velocities (Vs = 4.40-4.65 km/sec). This upper mantle structure is unique in the circum-Tyrrhenian area (Panza et al. 2004 Chap. 10). 

Walck M. C. (1984) The P-wave upper mantle structure beneath an active spreading center the Gulf of California. Geophys. J. Roy. Astro. Soc. 76, 697-723. 

Drummond B. J. (1988) A review of crust upper mantle structure in the Precambrian areas of Australia and implications for Precambrian crustal evolution. Precamb. Res. 40(1), 101-116. 

Luosto U., Tiira T., Korhonen H., Azbel L, Burmin V., Buyanov A., Kosminskaya 1., lonkis V., and Sharov N. (1990) Crust and upper mantle structure along the DSS Baltic profile in SE Finland. Geophys. J. Int. 101, 89-110. 

VanDecar, j. C. 1991. Upper-mantle structure of the Cascadia subduction zone from non-linear teleseis-mic travel-time inversion. PhD thesis. University of Washington, Seattle. 

These tests demonstrate the sensitivity of the higher-mode data to the upper-mantle structure. Figure 4a shows the displacement amplitude v. depth for the fundamental and first eight higher Rayleigh modes at 15 s period for the southern African model shown at the left in the figure. The fundamental mode at this period is sensitive to the velocity structure in the top c. 50 km of the model, whereas the higher modes are... 

Debayle, E. Kennett, B. L. N. 2000a. The Australian continental upper mantle structure and deformation inferred from surface waves. Journal of Geophysical Research, 105, 25423-24450. 

Jordan, T. H. Frazer, L. N. 1975. Crustal and upper mantle structure from Sp phases. Journal of Geophysical Research, 80, 1504-1518. 

R.L. Woodward and G. Masters (1991) Upper mantle structure from long-period differential travel times and free oscillation data. Geophys. J. Int. 109, 275-293. 

Eaton, D.W. Hope, J. 2003. Structure of the crust and upper mantle of the Great Slave Lake shear zone, northwestern Canada, from teleseismic analysis and gravity modelling. Canadian Journal of Earth Sciences, 40, 1203-1218. 

The variety of symmetries in the garnet structure (coordinations 4, 6, and 8) allows considerable compositional range. Table 5.16 lists the elements commonly present in positions X, X and Z. The diadochy of Al, Ti" ", and Fe in the tetrahedral site has been confirmed by Mossbauer spectroscopy on natural Fe-Ti-bearing garnets (Schwartz and Burns, 1978), and the presence of phosphorus in these sites, observed in upper mantle garnet, is attributable, according to Bishop et al. (1976), to coupled substitutions of the type... 

In the secondary distribution of the elements in the Upper Mantle and Crust, Goldschmidt believed that during the formation of a crystalline phase the principal factor controlling the behaviour of an element is the size of its ions. From tables of ionic radii one could predict the crystal structures in which a given ion was most likely to occur. This concept has immense qualitative value in crystal chemistry, and has been remarkably successful in view of the assumptions... 

The olivine spinel phase transition Experimental phase equilibrium studies have confirmed deductions from seismic velocity data that below 400 km, olivine and pyroxene, the major constituents of Upper Mantle rocks, are transformed to denser polymorphs with the garnet, y-phase (spinel) and P-phase (wadsleyite) structures (fig. 9.2). In transformations involving olivine to the P- or y-phases, transition pressures... 

Composition and mineralogy of the Mantle. The Earth s Mantle consists of Upper and Lower regions separated by the Transition Zone at depths between about 350 km and 650 km. Several phase changes occur in the Transition Zone in which common ferromagnesian silicates of the Upper Mantle, all containing Fe2+ ions in distorted six-coordinated sites and tetrahedrally coordinated Si, transform to dense oxide structures with cations occupying regular octahedral... 

Surrounding the core, the mantle has a thickness of about 2900 km. Its mass is estimated at 4 x 1024 kg. It is composed mainly of high-density silicates of Mg and Fe. It is divided into three layers lower (2000 km), transition (500 km), and upper mantle (360 km). The lower mantle is predominantly formed by Mg-perovskite, Mg-wurstite, and Ca-perovskite, which contain water in their crystal structures. Incredibly as it may seem, because of this water content the lower mantle is believed to contain more water than the oceans. 

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... 

Agee C. B. (1998) Phase transformations and seismic structure in the upper mantle and transition zone. Rev. Mineral. 165-203. 

Saltzer R. L. and Humphreys E. D. (1997) Upper mantle P wave velocity structure of the eastern Snake River Plain and its relationship to geodynamic models of the region. 

---