Relationship between fluid-bearing and fluid-absent invariant points and a petrogenetic grid for a greenschist facies assemblage in the system CaO-MgO-Al₂O₃-SiO₂-CO₂-H₂O

Abstract

In the 6 component system CaO-MgO-Al₂O₃-SiO₂-CO₂-H₂ with 9 solid phases (quartz, plagioclase, epidote, tremolite, talc, chlorite, magnesite, calcite, dolomite) and a fluid phase, all 17 possible fluid-absent reactions have been set up and balanced. Using molar entropy and volume data for the solid phases, these reactions are arranged in P-T space about the 8 possible fluid-absent invariant points after the method of Schreinemakers. Field observations in Ordovician greenschist facies basic volcanics at Sofala N.S.W., indicate that neither talc+epidote nor magnesite+calcite are stable under the conditions of metamorphism. Assuming these conditions to apply to the theoretical study here, the fluid-absent invariant points are arranged in a relative fashion with fluid-absent reactions subdividing P-T space into smaller areas.

A scheme which permits a fluid of composition $0 \leqq X_{{\text{H}}_{\text{2}} {\text{O}}} + X_{{\text{CO}}_{\text{2}} } \leqq {\text{1}}$ (i.e. a fluid containing CO₂ and H₂O together with other components), is modeled by treating H₂O as a mobile component independent of CO₂, and by allowing $U_{{\text{H}}_{\text{2}} {\text{O}}} - U_{{\text{CO}}_{\text{2}} }$ values that lie off the $U_{{\text{H}}_{\text{2}} {\text{O}}} - U_{{\text{CO}}_{\text{2}} }$ locus of binary H₂O-CO₂. Taking into account that neither talc+epidote nor magnesite +calcite is to be permitted, the fluid scheme is used to set up and balance all 39 possible fluid-bearing reactions. These are then arranged about 20 valid fluid-bearing invariant points in $U_{{\text{H}}_{\text{2}} {\text{O}}} - U_{{\text{CO}}_{\text{2}} }$ space after the method of Korzhinskii and Sehreinemakers.

A characteristic solid phase assemblage is defined for each P-T area using chemographic relations inherent from the fluid-absent boundary reactions. The fluid-bearing invariant points that have a solid assemblage compatible with the characteristic assemblage in a particular P-T area are stable within the P-T regime of that area. When these stable fluidbearing invariant points are arranged in a relative fashion in $U_{{\text{H}}_{\text{2}} {\text{O}}} - U_{{\text{CO}}_{\text{2}} }$ space, they outline a fluid grid which can be used to study the possible effects of local variation in X _fluid over the particular P-T regime.