Perovskite lower mantle composition

Wolf, G. H., and M. S. T. Bukowinski (1987). Theoretical study of the structural properties and equations of state of MgSiOj and CaSiOj perovskites implications for lower mantle composition. In High Pressure Research in Mineral Physics (M. Manghnani and Y. Syono, eds.) Tokyo Amer. Geophys. Union/Terra Pub. Co. 

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). 

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). 

Murakami et al. (2002) studied a natural peridotite composition (with 7.5-13.5 wt.% H2O) at 25.5 GPa and 1,600-1,650 °C. They measured water contents in their run products by SIMS. They found magnesium-rich perovskite and ferropericlase to have —2,000 ppm H2O and calcium-rich perovskite to have —4,000 ppm H2O. A lower mantle consisting of 79 wt.% Mg-perovskite, 16 wt.% ferropericlase, and 5 wt.% Ca-perovskite could contain 2,100 ppm H2O, which when integrated over the mass of the lower mantle yields a reservoir —5 times greater than the oceans. They compare this to the transition zone, which can store nearly six oceans worth of water, despite its smaller volume, because of the greater solubility of water in wadsleyite and ringwoodite (—3.3 X 10" ppm and... 

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). 

Li, B. and Zhang, J., 2005. Pressure and temperature dependence of elastic wave velocity of MgSi03 perovskite and the composition of the lower mantle. Phys. Earth Planet. Interior, 151, 143-54. 

The perovskite structure is also of interest to mineralogists. A mineral with the perovskite structure of composition close to MgSiOs is believed to be the predominant mineral in the lower mantle (depths of about 600km) of the earth. The perovskite structure of MgSiOs is stable only at very high pressures. 

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... 

The fine details of lower mantle chemical and mineral composition models remain in dispute, however. Notwithstanding the apparent adequacy of the (Mg,Fe)Si03 perovskite plus magnesiowtlstite models of the lower mantle, petrological experiments on peridotite- like assemblages yield phases that contain Ca and A1 [5, 6] which, barring an improbable efficient seg-... 

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