Silicate perovskite

Taura H, Yurimoto H, Kurita K, Sueno S (1998) Pressure dependence on partition coefficients for trace elements between olivine and the coexisting melts. Phys Chem Min 25 469-484 Taura H, Yurimoto H, Kato T, Sueno S (2001) Trace element partitioning between silicate perovskites and ultracalcic melt. Phys Planet Earth Int 124 25-32... 

Radiative heat transport through olivine has been discussed extensively (e.g., Fukao et al., 1968 Shankland, 1970 Schatz and Simmons, 1972 Scharmeli, 1979 Shankland et al., 1979). The radiative thermal conductivity, Kt of forsteritic olivine increases with rising temperature and would contribute to heat flow in the Upper Mantle (Shankland et al., 1979). However, values of Kt for olivine are considered to be rather low to satisfactorily explain the dissipation of the Earth s internal heat by radiation and lattice conduction alone. Note, however, that Fe2 CF transitions in almandine, pyroxenes (M2 site) and, perhaps, silicate perovskites absorb strongly in the wavelength range 1,250 to... 

Jackson, W. E., Knittle, E., Brown Jr, G. E. Jeanloz, R. (1987) Partitioning of Fe within high-pressure silicate perovskite Evidence for unusual geochemistry in the lower mantle. Geophys. Res. Lett., 14, 224-6. 

Li, X. Jeanloz, R. (1990) Laboratory studies of the electrical conductivity of silicate perovskites at high pressures and temperatures. J. Geophys. Res., 95,5067-78. 

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. 

Following the widespread acceptance of the view that silicate perovskites may be major components of the lower mantle (see Jeanloz and Thompson, 1983), there have been a number of attempts to calculate the structure, elastic properties, and equations of state of these materials (Wolf and Jeanloz, 1985 Wolf and Bukowinski, 1985, 1987 Matsui et al., 1987 Hemley et al., 1987). A great deal of interest has also been generated in the crystal chemistry of perovskite-structure phases because of their high-temperature superconducting properties. 

Kato T., Ringwood A. E., and Irifune T. (1988a) Experimental determination of element partitioning between silicate perovskites, garnets and liquids constraints on early differentiation of the mantle. Earth Planet. Sci. Lett. 89, 123-145. 

Figure 1 Depth-varying phase proportions in a pyrolite model mantle after the manner of Ringwood (1989), Ita and Stixmde (1992), and Bina (1998h). Phases are (a) ohvine, (fi) wadsleyite, (y) ringwoodite, (opx) orthopyroxene, (cpx) clinopyroxene, (gt-mj) garnet-majorite, (mw) magnesiowiistite, ((Mg,Fe)-pv) ferromagnesian sihcate perovskite, and (Ca-pv) calcium silicate perovskite. Patterned region at base denotes likely heterogeneity near core-mantle boundary.

Perhaps one of the most important consequences of a peridotite composition for the upper mantle is that the phase transitions in olivine that are manifested as seismic discontinuities should exhibit thermally controlled variations in their depth of occurrence that are consistent with the measured Clapeyron slopes (Bina and Helffrich, 1994) of the transitions. In particular, the olivine-wadsleyite transition at 410 km should be deflected upwards in the cold environment of subduction zones while the disproportionation of ringwoodite to silicate perovskite and magnesiowiistite at 660 km should be deflected downwards, thereby locally thickening the transition zone. In anomalously warm regions (such as the environs of mantle plumes as described below), the opposite deflections at 410 and 660 should locally thin the transition zone. The seismically observed topography of 20-60 km on each of the 410 and 660 is consistent with lateral thermal anomalies of 700 K or less (Helffrich, 2000 Helffrich and Wood, 2001). 

Figure 3 Effects upon olivine phase equilibria near 410 km depth of low temperatures in subduction zones, for mineral thermodynamic parameters of Fei et al. (1991). Dark lines denote boundaries of subducting slab. Phases are (a) olivine, (P) wadsleyite, (-y) ringwoodite, (mw) magnesiowustite, and (pv) ferro-magnesian silicate perovskite. Note that the a — 3 transition near 410 km is first uplifted and then bifurcates into a strongly uplifted diffuse a — a + -y transition overlying a weakly uplifted sharp boundary (after Bina, 2002) (vertical resolution is 1 km).

Similar results have been reported by Mattern et al. (2002), using more recent equations of state for lower-mantle minerals and incorporating the solubility of alumina in silicate perovskite. They also used a three-layered slab model (midocean ridge basalt (MORE) over harzburgite over pyrolite), but with a MORE composition (Si/(Mg - - Fe) = 2.29) intermediate between our extreme end-members of the Helffrich et al. (1989) eclogite (1.65) and the Helffrich and Stein (1993) gabbro (2.58). 

Zhang J. and Weidner D. J. (1999) Thermal equation of state of aluminum-enriched silicate perovskite. Science 284, 782 -784. 

Kellogg et al. (1999), however, have suggested, on the basis of a transition in seismic heterogeneity observed at —1,600 km depth, the possibility of a very deep layer extending hundreds of kilometers above the core-mantle boundary. One possibility is that a relic layer of dense, primordial crystalline differentiates (e.g., magnesium- and calcium-silicate perovskite) may have remained buried in the deep lower mantle until the present. Such a layer is a potential storehouse for trace elements, including radioactive heat-producing elements, and potentially could provide an important reservoir for bulk silicate Earth chemical mass balance... 

Frost D. J. and Langenhorst F. (2002) The effect of AI2O3 on Fe—Mg partitioning between magnesiowiistite and magnesium silicate perovskite. Earth Planet. Sci. Lett. 199, 227-241. 

Kato T., Ohtani E., Ito Y., and Onuma K. (1996) Element partitioning between silicate perovskites and calcic ultra-basic melt. Phys. Earth Planet. Inter. 96, 201-207. 

Tschauner O., Zerr A., Specht S., RochoU A., Boehler R., and Pahne H. (1999) Partitioning of nickel and cobalt between silicate perovskite and metal at pressures up to 80 GPa. Nature 398, 604-607. 

---