Mineralogical constitution of the deep mantle

Abstract

The nature of the transition zone in the mantle between about 300 and 900 km is reviewed in the light of recent high-pressure experimental results. Under the P-T conditions existing in the mantle it is found that pyroxene will break down to a denser assemblage of olivine plus stishovite (the rutile polymorph of SiO2) around 400 km. In the vicinity of 600 km the olivine inverts to a spinel structure, with a further increase in density. Finally, between 900 and 1050 km, the spinel breaks down into denser, close-packed phases, which persist to the core boundary at 2900 km. Two transformations are possible: (a) spinel → periclase + stishovite, or (b) spinel → periclase + MgSiO3 (corundum structure). The second reaction is considered more probable. The total increase in density caused by these phase changes between 400 and 1000 km is 0.6 to 0.7 g/cm3 (referred to zero pressure). The effect of less abundant components in the mantle such as FeO, Al2O3, CaO, and Na2O is to form solid solutions with the principal phases. This causes each of the principal phase transitions to be spread over a substantial depth range. As a result, the increase of density between 400 and 900 km is continuous. These results verify Birch's hypothesis on the constitution of the mantle and are in close agreement with the Bullen model A density distribution.