Earth transition zone

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... 

From seismic evidence, the density and elastic properties of the Earth are known to change markedly at the boundary between the mid-mantle (the transition zone) and the lower mantle. The 670-km discontinuity has been attributed to a pressure-induced transformation of (Mg,Fe)2Si04 to a fine mixture of the above silicate perovskite and magnesio-wiistite. 

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-... 

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. 

Frost D. J. (1999) The stability of dense hydrous magnesium sihcates in Earth s transition zone and lower mantle. In Mantle Petrology Field Observations and High Pressure Experimentation A Tribute to Francis R. (Joe) Boyd, Geochemical Society Special Publication No. 6 (eds. Y. Fei, C. M. Bertka, and B. O. Mysen). Geochemical Society, Houston, pp. 283 —296. 

Konzett J. and Fei Y. (2000) Transport and storage of potassium in the Earth s upper mantle and transition zone an experimental study to 23 GPa in simplified and natural bulk compositions. J. Petrol. 41, 583-603. 

Richard G., Monnereau M., and Ingrin J. (2002) Is the transition zone an empty water reservoir Inferences from numerical model of mantle dynamics. Earth Planet. Sci. Lett 205, 37-51. 

C. R., 11, Seifert F., and Woodland A. B. (1993) Ferric iron in the upper mantle and in transition zone assemblages implications for relative oxygen fugacities in the mantle. In Evolution of the Earth and Planets, Geophysical Monograph 74 (eds. E. Takahashi, R. Jeanloz, and... 

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. 

The Earth s mantle is peridotitic in composition and is significantly depleted in silica relative to primitive chondrites. Seismological evidence shows that the mantle is layered and can be divided into an upper and lower mantle, separated by a transition zone at 400-660 km depth. Above the transition zone the mantle is dominated by olivine and orthopyroxene with minor garnet and clinopyroxene. The lower mantle is made up of phases Mg- and Ca-perovskite and magnesiowustite. Seismic velocity contrasts between the upper and lower mantle are thought to reflect the ph ase transformations between the two and are not related to differences in bulk chemical composition. The lower mantle is separated from the outer core by the D" layer, a hot thermal boundary layer of enigmatic composition. 

FIGURE 3.2 Tomographic image of the Earth s mantle beneath the Japanese Arc, down to the core-mantle boundary showing the distribution of slow and fast seismic waves. The wave velocity distribution also reflects temperature distribution and shows the penetration of a cold subducting slab through the transition zone into the lower mantle (after Fukao et al., 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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