Phase transitions and mantle discontinuities

Abstract

Density and elasticity data are consistent with, but do not require, a uniform upper‐ and lower‐mantle composition. Such data cannot at present resolve the small changes in physical properties that would be required to keep distinct mantle reservoirs dynamically separated. Alternatively, the issue of chemical stratification in the mantle can be addressed by analyzing the details of the phase equilibria of the appropriate minerals. In order to do this we construct an internally consistent pressure calibration scale which is based in part on the coesite‐stishovite transition pressure Ptr(GPa) = 7.45(±0.3) + 2.1(±0.3) × 10−3T(°C) according to our reanalysis of the available data. We conclude that a discontinuous reaction occurs in the olivine component (α+γ = α+β) of the mantle at the conditions of the 400‐km seismological discontinuity; however, no discontinuous reactions corresponding to the 670‐km discontinuity have yet been identified. The only reaction observed in diamond cell experiments at approximately the pressures existing at 670‐km depth, the breakdown of γ‐spinel to form a silicate perovskite assemblage, appears not to satisfy the observed sharpness of this discontinuity. Thus it may be necessary to invoke either a univariant reaction that has not yet been observed experimentally or a chemical discontinuity at this depth. As the mantle is likely to be at temperatures higher than those of the experiments (estimated to be near 1000°C), additional univariant reactions involving the silicate ilmenite or garnet phases that are predicted to occur may be significant. One possible interpretation is that the mantle is of uniform composition and that such (hypothetical) reactions produce the 670‐km discontinuity. This would imply that the phase assemblages so far studied in high‐pressure‐temperature experiments are not those occurring in the mantle. Alternatively, the mantle could be chemically stratified.

Composition of the upper mantle: Geophysical tests of two petrological models

“…At the bottom of the transition region all pyroxene is present as mj; below 670 km 1' and gt-mj transform into pv-bearing assemblages. Although the phase relations for mineral compositions appropriate to our pyrolite and piclogite assemblages are not well determined, the above sequence of phase transformations is consistent with the results of J eanloz and Thompson (1983), Akaogi and Akimoto (1979) and Liu (1980). An important point is that di and jd are stable over a broader pressure range than opx.…”

“…However, it remains to be demonstrated that ol-rich assemblages provide either a unique or the most satisfactory fit to the seismic data. Anderson (1976Anderson ( , 1979, Liu (1979), Lees et al (1983), and Jeanloz and Thompson (1983) have discussed the difficulties of explaining the discontinuity at 670 km and the velocities in the transition region on the basis of phase relations in the ol and opx systems. A conclusion of the above studies is that the 670 km discontinuity may represent a chemical boundary.…”

Thermodynamic mixing properties of olivine derived from lattice vibrations

“…Table 4 indicates that our analyses prefer the ambient volume of Zhang et al (1998), Richard and Richet (1990), Hazen (1977) and Smyth (1975). This value is lower than the recommended value by Jeanloz and Thompson (1983) used in the analysis by Jacobs et al (2001). The V-P-T data of Yagi et al (1987), which are at the highest temperatures relative to all other measurements given in Table 2, are represented to within experimental uncertainty of about 0.3%.…”

“…Our analyses prefer the ambient volume of fayalite by Zhang et al (1998), Richard and Richet (1990), Hazen (1977) and Smyth (1975), which is lower than the recommended value given by Jeanloz and Thompson (1983). The difference between calculated volumes in pressuretemperature space produced by the two analyses is insignificant.…”

A note on latent heat release from disequilibrium phase transformations and deep seismogenesis

“…and called the "Verhoogen effect" (Jeanloz and Thompson, 1983). The T value given by the expression (5) for the Verhoogen effect simply reflects the conversion of the latent heat of equilibrium phase change ( H = T S) into temperature via the isobaric heat capacity.…”