Compositional variations of coexisting minerals have been considered important for understanding mineral reactions in aluminous granulites since pioneering experimental work of Hensen and Green (1971, 1972, 1973). Hensen and Harley (1990) in their graphical analysis of P-T-XMg (Mg/Mg+Fe2+) relations involving solid solutions of cordierite (Crd), garnet (Grt), orthopyroxene (Opx), sapphirine (Spr) and spinel (Spl), and end-member minerals sillimanite (Sil) and quartz (Qtz) in the system FeO-MgO-Al2O3-SiO2 outlined some effects of Fe and Mg partitioning on reaction boundaries and provided a sketch of the compositional changes. However, descriptions of the Fe-Mg distribution between the minerals, and in particular that related to sapphirine - as also provided for Spr-Crd and Spr-Opx by Grew (1980), for Spr-Opx and Spr-Spl by Higgins et al. (1979), for Spr-Spl by Owen and Greenough (1991) and for Spr-Crd by Waters (1986) - remained empirical. This work
is an attempt to analyze the phase relations by calculating equilibrium mineral compositions for certain assemblages under certain P-T-conditions from thermodynamic properties of the mineral reactions. The approach employed by Aranovich and Podlesskii (1989) was used to describe Crd-bearing reactions. Except for properties of reactions involving Fe-Spr-end-member, which were estimated empirically, the thermodynamic data were derived from available experimental data. Effects of oxygen fugacity were not considered.
The calculated P-T grid has four stable invariant points [Spl], [Opx], [Sil] and [Crd], with the topology being similar to that proposed by Hensen and Harley (1990), except that they indicated stability of [Qtz] instead that of [Crd], as implied by this work. Incorporation of fluid particles into channels of the cordierite structure has been estimated to increase the upper pressure stability limits of Crd-bearing assemblages from 5-6.5 kbar under fluid-absent conditions to 9.5-11.5 kbar under water-saturated conditions, which is 1-1.5 kbar lower than experimentally determined by Bertrand et al. (1991). The first three invariant points shift along (Crd) absent reaction curves towards higher temperatures under water-saturated conditions (from > 900ºC to > 1100ºC), thus coming closer to the most high-temperature high-pressure point [Crd]. This may imply that changes in fluid activity during metamorphism might have played an important role in forming significant diversity or reaction textures observed in aluminous granulites.
According to the derived relationships, XMg of coexisting phases increases in a row Grt, Spl, Opx, Spr, Crd, with Grt and Spl demonstrating inversions, and Spl becoming the most Fe-rich phase with increasing temperature. Coefficients of distribution of Fe and Mg between Spr and other minerals decrease with increasing temperature. The XMg of coexisting minerals jointly increase along univariant reaction curves for (Opx), (Sil) absent reactions towards invariant point [Spl], for (Qtz), (Spr) absent reactions towards invariant point [Opx], for (Opx), (Spr) absent reaction towards invariant point [Sil], for (Opx), (Sil), (Spr) absent reactions towards invariant point [Crd]. In other cases, XMg of coexisting minerals either decrease jointly along univariant reaction curves towards corresponding invariant points, or demonstrate more complex behavior, when maxima or minima of XMg occur for some minerals. Although, in general, XMg for Grt increases with increasing pressure, that is not a universal regularity.
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