Physics
Scientific paper
Nov 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991pepi...69...85k&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 69, Issue 1-2, p. 85-100.
Physics
7
Scientific paper
On the basis of phase diagrams of the FeO-MgO-SiO2 system and equations of state for minerals we have calculated internally consistent thermodynamic profiles of adiabatic bulk modulus, seismic parameter and density for a series of petrological models. It is shown that the sequences of phase assemblage changes across the 400 km discontinuity and the thickness of divariant transitions depend strongly on the iron content, the Ol/Px ratio and the temperature at the transition depth. Calculations show that the iron-rich model is inadequate to described the seismic properties in the uppermost mantle and through the 400 km discontinuity. The distribution of physical properties in the iron-rich model at the depths of 360-400 km is of a gradient type which does not agree with the seismic evidence; an important constraint on the upper-mantle chemistry is an Fe/(Fe + Mg) ration less than 0.17. Comparisons of calculated thermodynamic properties for various temperature and bulk composition models indicate that thermodynamic profiles of the olivine and pyroxene models do not match seismic observations through the 400 km discontinuity. Hence, the SiO2 content in the isochemical upper mantle should be in excess of 36.4 mol.% and less than 44.4 mol.%. From all the petrological models considered, only a pyrolite model (SiO2 = 40 mol.%, Fe/(Fe + Mg) = 0.12, Ol/Px = 1) may satisfy available seismological constraints. The results obtained indicate that seismic properties across the 400 km discontinuity might be consistent with an isochemical pyrolite composition and do not require chemical stratification. The pyrolite models considered show that the phase transitions occur over a range of 8-15 km at a depth near 400 km. It is proposed that in the different tectonic regions of the Earth a 400 km discontinuity might be considered as a combination of first- and second-order discontinuities with a phase transition sequence α + Px --> α + γ + Px --> α + β + Px --> β + Px within a thickness range of around 15 km (low-temperature pyrolite model) or as pseudo-first-order discontinuity through a narrow divariant loop α + β + Px about 8 km wide (high-temperature pyrolite model).
Kuskov Oleg L.
Panferov A. B.
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