Mantle transition zone

Akimoto S., Matsui S., and Syono S. (1976). High-pressure crystal chemistry of orthosilicates and the formation of the mantle transition zone. In The Physics and Chemistry of Minerals and Rocks, R. I G. Strens, ed. New York John Wiley. 

The properties of upper-mantle seismic reflectors, especially the observed lateral variations in the seismological properties of the mantle transition zone, indicate that the upper mantle possesses a peridotite composition, approaching... 

Deuss A. and Woodhouse J. H. (2001) Seismic observations of splitting of the mid-mantle transition zone discontinuity in Earth s mantle. Science 294, 354-357. 

Niu F., Solomon S. C., Silver P. G., Suetsugu D., and Inoue H. (2002) Mantle transition-zone structure beneath the South Pacific Superswell and evidence for a mantle plume underlying the Society hotspot. Earth Planet. Sci. Lett. 198, 371-380. 

Green H. W., II, Dobrzhinetskaya L., Riggs E. M., and Jin Z.-M. (1997) Alpe Arami a peridotite massif from the mantle transition zone Tectonophysics 279, 1-21. 

Kudoh Y., Inoue T., and Arashi H. (1996) Structure and crystal chemistry of hydrous wadsleyite, Mgi 758 0.504 possible hydrous magnesium silicate in the mantle transition zone. Phys. Chem. Mineral. 23, 461-469. 

Anderson D. L. (1979) Upper mantle transition zone— Eclogite. Geophys. Res. Lett. 6, 433-436. 

Fukao Y., Obayashi M., Inoue H., and Nenbai M. (1992) Subducting slabs stagnant in the mantle transition zone. J. Geophys. Res. 97, 4809-4822. 

Kelemen P. B., Koga K., and Shimizu N. (1997) Geochemistry of gabbro sills in the crust-mantle transition zone of the Oman Ophiolite implications for the origin of the oceanic lower crust. Earth Planet. Sci. Lett. 146, 475-488. 

There are, however, interesting details that are to date unique to the Western Superior Provinee. These include thicker than normal Archaean crust, a slab-like velocity anomaly in the mantle transition zone, and large SKS splitting in the Archaean Superior Province but little spUtting in the surrounding Trans-Hudson Proterozoic shear zone. 

The comparison at 2106 km distance (Fig. 3c) shows an acceptable match of the synthetic and observed waveforms with the base of the lid at 160 km depth. Thicker lids advance the arrival time of the wavepacket, but the waveform shape is not altered. At this distance range, the higher modes are equivalent to an S wave turning in the mantle transition zone. Increasing the lid thickness reduces the delay time caused by the lower shear-wave velocities beneath the lid, resulting in an advance in the arrival time but no change in the amplitude at this epicentral distance. 

The high-pressure mineral y-Mg2Si04 (a form of olivine found in the deep part of the mantle transition zone. Ringwoodite can contain water in its lattice A rock, such as peridotite which is dominated by dark, iron-magnesium-rich minerals such as olivine and pyroxene... 

Of particular significance are the density increases which take place at the upper and lower boundaries of the mantle transition zone, at the 410 km and 660 km discontinuities. In the past it has been argued that the large density increase which takes place at 660 km depth reflects a change in the bulk composition of the mantle with depth. However, the present consensus is that the contrasts can be accommodated simply by phase changes in the mantle mineralogy. (This debate has huge consequences for whether or not the mantle is chemically layered, and is an important factor in the current debate about the nature of mantle convection). 

At about 500 km depth, within the mantle transition zone, olivine undergoes a further phase change from /3-Mg2Si04 to spinel structured Mg2Si04 - y-Mg2Si04 (ringwoodite - Fig. 3.1). 

The water filter model implies that the source of midocean ridge basalts is the lower mantle and that it is depleted in the transition zone, as it rises. It adequately explains the geochemical differences between MORB and OIB but still requires deeply sourced mantle plumes. One problem that this model does not resolve however, is that of the isotopic difference between MORB and OIB (see Fig. 3.5b). For the removal of trace elements in the mantle transition zone by small fraction melting by the "water filter process" does not alter isotope ratios and will not therefore impart to MORB and OIB their isotopic distinctiveness (Hofmann, 2003). 

In basalts, lawsonite is the only hydrous phase which is stable in the deep mantle and this phase dehydrates at the depth of the mantle transition zone. Similarly in subducted pelagic sediments there is a series of hydrated aluminum silicates and oxides (including the phase Egg - AlSiOs.OH) which also remain stable down to transition zone depths (Williams Hemley, 2001). 

FIGURE 5.1 Model of water cycling in the Earth (after Ohtani, 2005). UM, upper mantle TZ, mantle transition zone LM, lower mantle. The grey arrows show water-in to the mantle, the white arrows shown water-out of the mantle. The approximate lower stability limit of hydrous phases in the mantle is shown as phase-out. 

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