Abstract
The phase relations of orthopyroxene, olivine, and grunerite, in the presence of magnetite, are quantitatively evaluated using an ideal-solution model for the silicates. The evaluations are directly applicable to estimates of the physicochemical conditions of highgrade metamorphism of iron-formations and associated retrograde reactions.
Standard thermodynamic properties for ferrosilite and grunerite were calculated and estimated from published experimental and petrologic data. S°298, ΔH°f,298 and ΔG°f,298 of ferrosilite are 93.05±0.3 J/mol · K, -1194.872±1.0 kJ/mol, -1117.124± 1.0 kJ/mol, respectively; those of grunerite 721.8± 10.0 J/mol · K, -9648.304 ±20 kJ/mol (minimum), and -8988.759±20 kJ/mol (minimum), respectively.
The phase relations are evaluated on log fO2-T diagrams, at constant pressure. The stability field of orthopyroxene is restricted within a narrow range of fO2 in fayalite-bearing systems. Because of the compositional limit ( > 0.8) of olivine in iron-formations, an upper fO2 limit for olivine may be set. At the same P and T, orthopyroxene is stable at higher fO2 conditions than olivine. Consequently the phase relations for iron-formations may be classified into three types: (I) olivine-free, (II) olivine- and quartz-bearing, and (III) olivine-bearing and quartz-free relations. Each type may or may not include grunerite. Its presence is strongly influenced by the magnitude of fH2O. Grunerite in type (I) assemblages tends to have lower , and is related to higher fH2O than that of type (II).
The retrograde changes of (1) olivine → grunerite, (2) orthopyroxene → grunerite, and (3) olivine → orthopyroxene relate directly to the phase relations discussed in this study. Reactions (1) and (3) may never take place in assemblages of type (I) but may be observed in the rocks of types (II) and (III). The formation of retrograde grunerite appears to be dependent upon fH2O rather than on the lowering of T.
