Staurolite Stability and Related Parageneses: Theory, Experiments, and Applications

The results of recent investigations on the stability limitsof staurolite have been combined together with those of thepresent study to develop a semi-quantitative model of the P–T–f_o2–Xrelations of staurolite±quartz±magnetite. Theproblem with respect to the hydroxyl content of staurolite hasbeen analysed; it is concluded that no evidence has yet beenmustered to discount the idealised stoichiometry proposed byNaray-Szabó & Sasvari (1958), at least as a limitingcomposition. The stability limits of staurolite±magnetitehave been calculated from the experimental data for the equilibriainvolving quartz. Also the conditions over which the assemblagecordierite+magnetite+quartz could be stable, as well as a quantitativemodel for the fo₂-P stability of almandine ± quartz havebeen deduced theoretically. An analysis is presented of the paragenetic relations of staurolitein common pelitic schists. It is suggested that the formationof staurolite at the expense of either chloritoid or chlorite,rather than the unqualified first appearance of staurolite asproposed by Winkler (1970), should define a ‘staurolite-in’isograd in the range of 500–575 °C. In regional metamorphism,chloritoid, staurolite, and aluminum silicates should, underequilibrium conditions, be unstable relative to almandine ingraphitic pelitic schists involving magnetite (chloritoid/staurolite/Al₂SiO₆+magnetite+quartzalmandine+O₂+H₂O).The limits of P-T conditions over which staurolite and cordieritemay coexist in natural assemblages have been deduced; it isrestricted, almost entirely within the field of andalusite,between 500–700 °C, and 2–6 kbars, thus definingthe range of P-T conditions for the ‘low-pressure intermediate’—or ‘Buchan’–type amphibolite facies discussedby Miyashiro (1961). In assemblages involving staurolite andandalusite, cordierite rather than almandine should usuallybe stable; the reverse holds for assemblages involving stauroliteand sillimanite.     

Equilibrium Compositions of Coexisting Garnet and Orthopyroxene: Experimental Determinations in the System FeO-MgO-Al2O3-SiO2, and Applications

We have determined the Fe-Mg fractionation between coexistinggarnet and orthopyroxene at 20–45 kb, 975–1400?C,and the effect of iron on alumina solubility in orthopyroxeneat 25 kb, 1200?C, and 20 kb, 975?C in the FMAS system. The equilibriumcompositions were constrained by experiments with crystallinestarting mixtures of garnet and orthopyroxene of known initialcompositions in graphite capsules. All iron was assumed to beFe²⁺. A mixture of PbO with about 55 mol per cent PbF₂ provedvery effective as a flux. The experimental results do not suggest any significant dependenceof K_D on Fe/Mg ratio at T 1000?C. The lnK_D vs. l/T data havebeen treated in terms of both linear and non-linear thermodynamicfunctional forms, and combined with the garnet mixing modelof Ganguly & Saxena (1984) to develop geothermometric expressionsrelating temperature to K_D and Ca and Mn concentrations in garnet. The effect of Fe is similar to that of Ca and Cr³⁺ in reducingthe alumina solubility in orthopyroxene in equilibrium withgarnet relative to that in the MAS system. Thus, the directapplication of the alumina solubility data in the MAS systemto natural assemblages could lead to significant overestimationof pressure, probably by about 5 kb for the relatively commongarnetlherzolites with about 25 mol per cent Ca+Fe²⁺ in garnetand about 1 wt. per cent Al₂O₃ in orthopyroxene.