The constitution of the mantle—III Consequences of the olivine-spinel transition

Abstract

Evidence on the chemical composition of the mantle is reviewed. Based upon studies of the inclusions in kimberlite pipes, intrusion of peridotites in orogenic regions, and the compositions of stony meteorites and kimberlite, it is suggested that the average composition of the mantle may approximate that of a garnet peridotite. However, the upper mantle immediately below the M-discontinuity may be poorer in sialic material and close to normal dunite-peridotite in composition. When garnet peridotite is subjected to high temperatures and pressures a series of transitions dominated by the olivine—spinel transition occurs. Besides the olivine-spinel transition, pyroxene may invert to spinel plus coesite, whilst garnet and coesite may dissolve in the spinel at higher temperatures to form a highly disordered defect solid solution. The data in Parts I and II show that these changes should occur under the P- T conditions present in the upper 1000 km of the mantle and take place over an appreciable depth range. The position and extent of this transition range depends upon the assumed temperature distribution in the upper mantle. By choosing a suitable temperature distribution, the transition region can be made coincident with region C (Bullen) in which phase changes and inhomogeneity have been inferred on other grounds. It is found that the proposed transition phenomena are capable of explaining the known data on seismic velocity distribution, elasticity and density in the mantle. The temperature distribution required to make the phase transition region coincident with region C is supported by recent studies of the temperature dependence of electrical conductivity in the mantle. Both convection and conduction mechanisms of heat transfer may give rise to this temperature distribution. A specific model for the mantle is proposed.