PHASE TRANSFORMATIONS AND PARTITIONING RELATIONS IN PERIDOTITE UNDER LOWER MANTLE CONDITIONS

Abstract

The phase transformations which take place in peridotite between 19 and 25 GPa have been investigated by performing experiments at closely spaced pressure intervals. All reactions involving Mg-perovskite are influenced by its Al2O3 content which controls Mg-Fe2+-Fe3+ partitioning relationships between this phase and the other major lower mantle phase, magnesiowustite. At 1900K, (Mg,Fe)2SiO4 spinel breaks down at about 22.5 GPa to produce an Mg-perovskite low in Al2O3 (=<1%), and high in Mg (Mg*=Mg/[Mg+Fe2++Fe3+]#0.96), together with Fe-rich magnesiowustite (Mg*#0.82). The low Al2O3 content of the perovskite means that the partitioning relations are similar to those in the Al2O3-free system and that the reaction should be near-univariant, consistent with a sharp 660 km discontinuity. Garnet breaks down through the pressure interval 22.5-24 GPa producing Ca-perovskite and an (Mg,Fe,Al)-perovskite component which dissolves into the existing low Al2O3 Mg-perovskite. Garnet and the Mg-perovskite become richer in Al through the transformation interval and the Mg* of the latter declines due to the general coupling between Al and (Fe3++Fe2+). At pressures of 24-25 GPa, garnet disappears and perovskite can dissolve all of the Al2O3 present in mantle peridotite. The transformation of garnet to aluminous perovskite may be responsible for the abnormally high seismic velocity gradients at the top of the lower mantle (660-760 km depth). From the measured phase compositions, a peridotitic lower mantle would contain, by weight 79% (Mg,Fe,Al)-perovskite, 16% magnesiowustite and 5% Ca-perovskite.