Eutectoid phase transformation of olivine and spinel into perovskite and rock salt structures

Abstract

The 700-km seismic discontinuity which defines the top of the Earth's lower mantle is generally attributed to the transformation of the upper mantle (Mg, Fe)2 SiO4 spinel into magnesiowustite (Mg, Fe)O and (Mg, Fe)SiO3 with perovskite structure1. Large-volume high-pressure experiments2 have provided important thermodynamical information on this transition, but its mechanisms and kinetics are still unknown. Here we report microstructural observations of the products of the transformation of (Mg, Fe)2SiO4 olivine and spinel and Ca2GeO4 olivine, effected in a laser-heated diamond anvil cell (DAC). The observations were made by transmission electron microscopy (TEM), a technique that has already proved useful in investigating the olivine-spinel transition in the DAC3,4. We find that, in all cases, the transformed product has a eutectoid-like appearance (as in pearlitic carbon steels), with alternating lamellae of the rock salt and perovskite phases. The thickness and spacing of the lamellae increase with temperature until a granular structure forms. These observations suggest that the transformation starts as a eutectoid transformation, whose physical mechanisms might be as effective in the Earth's mantle as in the DAC.