Perovskite upper mantle composition

Three kinds of evidence have been put forward in support of a lower mantle with a different composition from the upper mantle. The first was the apparent lack of a match between the seismic and other geophysical properties observed for the lower mantle, and the laboratory-measured properties of lower mantle minerals (MgSi03-rich perovskite and magnesiowiistite) in an assemblage with the upper mantle composition (meaning, effectively, with the upper mantle s Mg/Si and Mg/Fe ratios). Jackson and Rigden (1998) reinvestigated these issues and conclude that there is no such mismatch (see Chapter 2.02). 

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). 

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... 

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. 

Composition of the lower mantle A variety of compositions have been proposed for the lower mantle including almost pure perovskite, chondrite, and pyrolite. However, most models of the Earth assume that the upper and lower mantle have the same composition. Recent attempts to directly estimate the composition of the lower mantle have used best-fit curves of the thermoelastic properties of the Earth to a PREM model mantle made up of the phases Mg-perovskite, Ca-perovskite, and magnesiowustite. The recent calculations by Li and Zhang (2005) indicate a pyrolitic composition for the lower mantle, with Mg/Si atomic ratios between 1.29 and 1.39, slightly higher than those for the pyrolite models in Table 3.1 (Mg/Siatomic = 1.24—1.25). 

The seismic discontinuity occuring in the earth s mantle at a depth of 670 km is attributed to the phase transition from the spinel phase to the perovskite phase, and it is this discontinuity which marks the separation between the upper and lower mantle. Calculations were carried out of this phase transition in the MgSi03 and Mg2Si04 systems, which approximately model the composition of the mantle... 

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