High-pressure polymorphs of olivine and the 660-km seismic discontinuity

Abstract

It had long been accepted that the 400-km seismic discontinuity in the Earth's mantle results from the phase transition of (Mg,Fe)₂-SiO₄-olivine to its high-pressure polymorph β-spinel (wadsleyite), and that the 660-km discontinuity results from the breakdown of the higher-pressure polymorph γ-spinel (ringwoodite) to MgSiO₃-perovskite and (Mg,Fe)O-magnesiowüstite^1,2,3,4. An in situ multi-anvil-press X-ray study⁵ indicated, however, that the phase boundary of the latter transition occurs at pressures 2 GPa lower than had been found in earlier studies using multi-anvil recovery experiments⁶ and laser-heated diamond-anvil cells⁷. Such a lower-pressure phase boundary would be irreconcilable with the accuracy of seismic measurements of the 660-km discontinuity, and would thus require a mineral composition of the mantle that is significantly different from what is currently thought. Here, however, we present measurements made with a laser-heated diamond-anvil cell which indicate that γ-Mg₂SiO₄ is stable up to pressure and temperature conditions equivalent to 660-km depth in the Earth's mantle (24 GPa and 1,900 K) and then breaks down into MgSiO₃-perovskite and MgO (periclase). We paid special attention to pressure accuracy and thermal pressure in our experiments, and to ensuring that our experiments were performed under nearly hydrostatic, inert pressure conditions using a variety of heating methods. We infer that these factors are responsible for the different results obtained in our experiments compared to the in situ multi-anvil-press study⁵.