Postspinel transformations and the width of the 670-km discontinuity: a comment on "Postspinel transformations in the system Mg2SiO4-Fe2SiO4 and some geophysical implications' by E. Ito and E. Takahashi

In their recent paper dealing with postspinel transformations in the system Mg₂SiO₄-Fe₂SiO₄, Ito and Takahashi [1989] showed that the reaction (Mg,Fe)₂SiO₄ = (Mg,Fe)SiO₃ + (Mg,Fe)O (spinel = perovskite + magnesiowustite) occurs over a very narrow pressure interval, at 1600°C, for values of Fe/(Fe+Mg) of between 0.0 and 0.3. It is the purpose of this comment to point out that the argument for a narrow transition interval can be independently confirmed and strengthened using Ito and Takahashi's data on Fe-Mg partitioning between the phases spinel, perovskite, and magnesiowustite. Ito and Takahashi's contention that the postspinel transition in (Mg,Fe)₂SiO₄ is sharp enough to produce the major, or sole, contribution to the 670 km discontinuity is reinforced. -from Author