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

Abstract

The elastic properties of candidate mantle phases are used to test the viability of olivine‐rich (pyrolitic) and CaO + Al_2O_3‐rich (eclogitic) assemblages for the mantle. High temperature adiabats for each phase of interest are constructed and compared to mantle seismic properties. Both pyrolitic and eclogitic assemblages satisfy the seismic properties between ∼200 and 400 km. Between 400 and 670 km depth an eclogitic assemblage yields a superior match to velocities and velocity gradients. The 400 km seismic discontinuity may represent a chemical boundary between pyrolite and picritic eclogite ("piclogite") or phase transformations in the olivine + orthopyroxene components of a piclogitic assemblage containing about 16% olivine. High velocity gradients in the transition zone may be explained by the transformation of Ca‐rich cpx to majorite garnet. Seismic properties at the top of the lower mantle are consistent with pyrolite, piclogite or perovskite, implying that the 670 km discontinuity may be a chemical boundary. Comparisons of laboratory elasticity data with seismic velocity profiles have been used in many studies to constrain the composition of the Earth's mantle. With few exceptions, it has been assumed a priori that the dominant minerals throughout the mantle are olivine (ol) and orthopyroxene (opx), coexisting with a small quantity of garnet (gt) and clinopyroxene (cpx). However, it remains to be demonstrated that ol‐rich assemblages provide either a unique or the most satisfactory fit to the seismic data. Anderson (1976, 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. Differences in the Fe and/or Si (opx) content of the upper and lower mantle have been suggested, and Anderson (1979) proposed that the mantle between 220–670 km depth is dominated by gt + cpx (eclogite) rather than ol + opx. It is clearly desirable to test the viability of the gt‐cpx hypothesis by direct comparison of the high pressure‐high temperature velocities for such an assemblage with mantle velocity and density profiles. Although the stability fields and elastic properties of CaO‐ and Al_2O_3‐rich high‐pressure phases are less well constrained than those of ol and opx, sufficient information exists for silicates and analog compounds to examine the plausibility of an eclogitic region. In this paper we calculate the compressional (V_p) and shear (V_s) velocities, and density (ρ) of olivine, pyroxenes, garnets, and their polymorphs, at elevated temperatures and pressures. The properties of assemblages dominated by gt + cpx or ol + opx are then compared to observed mantle velocities and densities.

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