EOSC 322 – Metamorphic Petrology
Spring 2025
PROBLEM SET #4 – Reactions in Siliceous Carbonates
DUE - Tuesday, March 11, 2025
This exercise is designed to illustrate (1) the usefulness of ternary diagrams in ‘visualizing’ metamorphic reactions and (2) that the composition of the fluid phase is an important variable in metamorphism, particularly for calcareous rocks. You are given a T-X(CO2) diagram (Figure 1) that shows a set of metamorphic reactions that occur in siliceous carbonates. All of the minerals can be plotted on a CaO-MgO-SiO2 ternary diagram, by projecting through the H2O-CO2 fluid phase (Figure 2). On Figure 1, I have illustrated the stable minerals and tie-lines (the so-called ‘phase topology’) of the highest and lowest temperature mineral assemblages. Your assignment is as follows:
1) Draw the stable mineral assemblages and tie-lines on the ternary diagrams provided (Figures 3 and 4) for each of the isobaric divariant fields labelled 1 through 12. Start from the ternary diagrams given at the lowest and highest temperatures and work towards the middle. The first sequence (1 to 7) represents the sequence of prograde metamorphic reactions expected in siliceous carbonates coexisting with H2 O-rich fluid (Figure 3). The second sequence (8 to 12) represents the prograde sequence for CO2-rich fluids (Figure 4). Please use a ruler (straight edge) to construct tie-lines in each ternary diagrams.
2) Reaction ‘F’ on the T-X(CO2) diagram is labelled with question marks. You should be able to determine the reaction by examining the ternary diagrams on either side of the reaction as deduced in question (1). What is this reaction? Write the (unbalanced) reaction as it occurs with increasing temperature in the space below:
Figure 1. Temperature - X(CO2) for carbonaceous rocks. Temperature increases upward and to X(CO2) increases to the right.
Figure 2. SiO2-CaO-MgO ternary phase diagram for carbonaceous rocks. Note the various co- linear phases along the coloured lines as well as the along the edges of the SCM diagram.
Figure 3. Ternary phase diagrams for the assemblages and reactions in siliceous carbonates for H2O-rich fluids
Figure 4. Ternary phase diagrams for the assemblages and reactions in siliceous carbonates for CO2-rich fluids