Staurolite Stability in a Part of the System Fe-Al-Si-O-H

Abstract

The following reactions, believed to be analogous to those which define the maximum extent of staurolite-quartz compatibility at moderate oxygen fugacity in metamorphic rocks, have been determined in terms of hydrous fluid pressure and temperature. The O: H composition ratio of the fluid was controlled with the quartz-fayalite-magnetite (QFM) buffer assemblage. (I) Fe-staurolite÷quartz ⇌ almandine+sillimanite+water. (II) Fe-staurolite÷quartz ⇌ Fe-cordierite+sillimanite+water. (III) Fe-chloritoid+sillimanite ⇌ Fe-staurolite+quartz+water. In addition, two reactions which delineate part of the stability limits of Fe-cordierite have been investigated: (IV) Fe-cordierite ⇌ almandine+sillimanite+quartz. (V) Fe-cordierite ⇌ hercynite+sillimanite+quartz. The experimental information has been used to predict boundaries to the PT fields of all quartz and QFM-buffered fluid-bearing assemblages involving Fe-staurolite, Fe-cordierite, Fe-chloritoid, almandine, and sillimanite. Using information from this and other studies, three mineral assemblages are recognized which are stable at similar temperatures but different fluid pressures. In order of decreasing pressure they are: (a) Above ∼5 kb: staurolite, quartz, kyanite, fluid; (b) Between ∼1.5 and ∼8.5 kb (outer limits; in natural rocks this field will have a much narrower pressure range) staurolite, quartz, cordierite, fluid. (c) Below ∼3.5 kb: Fe-cordierite, andalusite, fluid of oxygen fugacity equivalent to the quartz-fayalite-magnetite assemblage. These phase assemblages may be the equivalents of naturally occurring mineral facies, but this must be proven in the field. In addition the absence of cordierite from rocks of appropriate composition and temperature of formation betokens total pressures greater than 3–5 kb.