Mantle seismic discontinuities

Helffrich G. and Bina C. R. (1994) Frequency dependence of the visibility and depths of mantle seismic discontinuities. Geophys. Res. Lett. 21, 2613-2616. 

Kind R., Li X., Yuan X., Sobolev S. V., Hanka W., Ramesh S. D., Gu Y. G., and Dziewonski A. M. (2002) Global comparison of SS precursor and Ps conversion data from the upper mantle seismic discontinuities. Eos, Trans., AGU 83(47), Pall Meet. Suppl., Abstract S52C-07. 

The chemical composition of the Lower Mantle below 670 km is essentially unknown. It has often been assumed to be the same as the Upper Mantle with the seismic discontinuity at 670 km representing a phase change to denser polymorphs rather than a chemical boundary (Liu and Bassett, 1986). However, some models of the Earth s interior suggest that the Mantle is stratified with the Upper Mantle and Lower Mantle convecting separately, leading to compositional density differences between these two regions. There is a commonly held view that the Lower Mantle has a higher Fe/(Mg+Fe) ratio than the Upper Mantle (Liu and Bassett, 1986 Jeanloz and Knittle, 1989). 

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). 

Given the opposing signs of the Clapeyron slopes of the primary phase transitions associated with these seismic discontinuities, any elevated mantle temperatures associated with thermal plumes may be expected to yield thinning of the transition zone (Figure 2), via depression of the 410 and uplift of 660 (Shen et al., 1998 Bina, 1998c Lebedev et al., 2002). Some global and... 

Irifune T. and Isshiki M. (1998) Iron partitioning in a pyrolite mantle and the nature of the 410-km seismic discontinuity. Nature 392, 702-705. 

With the exception of Davies, who favored whole-mantle convection all along, the above authors concluded that it was only the upper mantle above the 660 km seismic discontinuity that was needed to balance the continental crust. The corollary conclusion was that the deeper mantle must be in an essentially primitive, nearly undepleted state, and consequently convection in the mantle had to occur in two layers with only little exchange between these layers. These conclusions were strongly reinforced by noble gas data, especially He/ He ratios and, more recently, neon isotope data. These indicated that hotspots such as Hawaii are derived from a deep-mantle source with a more primordial, high He/" He ratio, whereas MORBs are derived from a more degassed, upper-mantle reservoir with lower He/ He ratios. The noble-gas aspects are treated in Chapter 2.06. In the present context, two points must be mentioned. Essentially all quantitative evolution models dealing with the noble gas evidence concluded that, although plumes carry... 

There are several possible descriptions of a layered mantle. The possibilities that have been incorporated into noble gas models include a boundary layer at the 670 km seismic discontinuity, a deeper layer of variable thickness, and a boundary layer at the core-mantle boundary. 

Hirose K., Fei Y., Ono S., Yagi T., and Funakoshi K.-I. (2000) In situ measurements of the phase transition boundary in Mg3Al2Si30i2 implications for the nature of the seismic discontinuities in the Earth s mantle. Earth Planet. Sci. Lett. 5705, 1-7. 

Chapter 2.02 concludes that within broad bounds, an isochemical mantle with composition similar to that inferred from upper mantle rocks is consistent with the seismic properties of the whole mantle. This includes not only wave speeds, but also the appearance of seismic discontinuities at 410 km and 660 km depth that mark the expected phase transformations as upper mantle olivine changes first into spinel and then into perovskite structured high-pressure polymorphs. As discussed in Chapter 2.02, because the phase diagrams for these mineralogical transformations can be determined in the laboratory, the exact depth of the corresponding seismic discontinuities... 

Heat flow data provide important constraints on mantle models. For example, combined with the heat producing element content of crust and mantle rocks, and the physical properties of mantle minerals, they can be used to constrain the nature of thermal convection in the mantle. In addition, variations in the mantle contribution to crustal heat flow between the continents and oceans have been used to make inferences about the nature of the different types of mantle underlying continental crust and oceanic crust (Section 3.1.2 and Chapter 4, Section 4.3.1.2). Furthermore, heat flow data, combined with bathymetric measurements, rates of sea-floor subsidence, and the depth of seismic discontinuities are all a function of mantle temperature and can be used to estimate relative, lateral variations in mantle temperature (Anderson, 2000). 

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