The sequence of high-pressure phase transitions α-olivine [RIGHTWARDS ARROW] β-modified spinel [RIGHTWARDS ARROW] γ-spinel is commonly used as a model for upper mantle seismic velocity increases in the 200–650-km depth interval. The widths of seismic transition zones and the corresponding divariant (e.g., α+β and β+γ) mineral stability fields are important criteria for correlating velocity variations with phase changes. Divariant mineral stability fields are poorly known for mantle molar Mg/(Mg+Fe) ratios (about 0.9), but C.R. Bina and B.J. Wood of the Department of Geological Sciences, Northwestern University (Evanston, Ill.), have demonstrated that these fields can be constrained by requiring the Mg2SiO4 -Fe2SiO4 phase diagram to be consistent with known thermochemical data. They have derived an internally consistent phase diagram (Figure 1) based on available calorimetric, thermoelastic, and synthetic data. They find that the divariant transition a [RIGHTWARDS ARROW] α + β [RIGHTWARDS ARROW] β , which is generally regarded as occurring over a broad depth interval, is in fact extremely sharp. For mantle olivine compositions the transition takes place over a pressure interval of only about 2 kbar. The sharpness of this transition, claim the authors, is quite insensitive to uncertainties in the constraining thermodynamic data.