Phase transformations in the mantle

Abstract

This paper presents a review of the constitution of the mantle between depths of 200 and 1200 km in the light of recent experimental investigations on high pressure phase transformations in silicate minerals and on their germanate analogues. Phase transformations occurring at depths to 600 km can be studied directly using apparatus which develops >200 kb at elevated temperatures. These studies indicate that the rapid increase of seismic velocity around 350–450 km is caused mainly by the transformation of pyroxenes into a new type of garnet structure and the transformation of olivine to the spinel (or related) structure. Current understanding of the constitution of the mantle below 600 km rests heavily upon interpretation of phase transformations in germanate analogue systems and on shock wave investigations. These suggest that around 600–700 km, garnets and spinels transform to new phases possessing ilmenite, perovskite and strontium plurabate structures with densities and clastic properties resembling those of isochemical mixed oxides (MgO + SiO2 (as stishovite) + Al2O3 + CaO, etc.). At greater depths, a further set of transformations to new phases appears likely, causing the “zero-pressure density” of the mantle to attain values about 5% higher than an isochemical mixture of oxides. The phase transformations which have been directly observed or inferred provide a quantitative explanation of the seismic velocity distribution between 300 and 800 km. From a study of the elastic properties of the mantle it is concluded that the average FeO/(FeO + MgO) ratio of mantle silicates is probably approximately uniform at 0.11 (molecular) throughout the entire mantle.