Physics
Scientific paper
Jun 1973
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1973pepi....7..187a&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 7, Issue 2, p. 187-198.
Physics
4
Scientific paper
The relative distribution of Fe2+ and Mg2+ between mantle minerals is calculated for various mantle thermal models and is used to estimate compressional and shear velocity and density profiles to depths of 670 km using ultrasonic, equation of state, and thermodynamic data for olivines, pyroxenes, garnets, and spinels. Except for uncertainties, arising from the lack of a quantitative description of the low-velocity zone, and inadequate elasticity data for Ca-bearing minerals, especially the clinopyroxenes, the calculated seismic profiles for pyrolite and lherzolite upper mantles are virtually identical. Varying the total iron content ratio, Fet = Fe/(Fe + Mg) from 0.1 to 0.2, brackets the seismic observation Important constraints to the composition and thermal profile of the mantle underlying continental regions are obtainable by fitting theoretical models to the detailed Vp-profile of Helmberger and Wiggins. This profile, although possibly inaccurate in the vicinity of the low-velocity zone, is well determined below 350 km. Values of Fet = 0.15 - 0.5 with a pyroxene-garnet (pyrope-almandine, majorite, solid-solution) content of 25 +/- 10% and a relatively cool geotherm (1630-1650°C, 570-670 km) are implied from the position of the α (olivine) to α + β to β (modified spinel) transition, at a depth centered at ~420 km, the position of the β to β + γ to γ (spinel) transition, centered at ~500 km as well as the absolute value of the P-wave velocity. Because of the low resolution of the VS- and ρ-profiles, presently offered by seismic techniques, the concomitant VS- and ρ-profiles are useful as predictive, rather than comparitive, tools. If, as has been suggested by Anderson and co-workers, substantial enrichment of iron occurs with increasing depth in the mantle, this study demonstrates that for at least the continental regions, such an increase, if it occurs, would take place at, or below the 670-km discontinuity in the mantle. Distribution coefficients, based on thermochemical data, predict that within the upper 100 km of the mantle, successively greater relative iron contents will be present in the order: orthopyroxene, clinopyroxene, olivine, and garnet. The distribution of Fe2+ between orthopyroxene and the coexisting mantle phases is calculated (using the Mössbauer data of Virgo and Hafner for the M1, M2 intersite energy in the formulation of Grover and Orville). The relative enrichment of iron in orthopyroxene (relative to clinopyroxene) in the upper 100 km decreases rapidly with depth. Below 100 km, orthopyroxene composition becomes insensitive to the assumed intersite energy. In the upper 100 km Fe2+ is markedly concentrated in the garnet phase. For a total mantle value of Fet of 0.15 the garnet will have a composition corresponding to Fe/(Fe + Mg) = 0.30. At a depth of about 100-150 km the Fe2+ content of the garnet decreases and approaches that of the bulk mantle. Below this depth range, the relative content of iron in the garnet again increases.
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