6.2 Geothermometry in the Field

Als Phase Diagram

Figure 6.03. Al2SiO5 stability diagram. Equilibrium Temperature (T)-Pressure (P) stability of three crystalline phases (polymorphs) with the chemical composition Al2SiO5: andalusite (And), kyanite (Ky), and sillimanite (Sil). Click on the image to see a larger version with more information.

Some quantitaive information about temperatures (T) and pressures (P) of metamorphism can be gathered by identifying the minerals or mineral assemblages of metamorphic rocks. The aluminosilicate (Als) minerals andalusite, kyanite, and sillimanite are star performers in this regard. These minerals commonly occur in metamorphosed shales and can be recognized in the field. The three aluminosilicate minerals all have the same chemical composition, but different crystal structures. They are polymorphs in a one-component system (Al2SiO5), and at equilibrium only one polymorph is stable at a time except at the special conditions shown by the reaction curves in Figure 6.03 where two polymorphs can be stable.

The three aluminosilicate reaction curves meet at a "triple point" at temperatures and pressures within the medium P/T (Barrovian) facies series of regional metamorphic rocks. The intersection of the reaction curves at metamorphic conditions is especially valuable. For example, if sillimanite is observed in a metamorphic rock, that rock must have been heated above the triple point temperature (545°C). Similarly, if in a prograde sequence the mineral kyanite is replaced by sillimanite, the metamorphic pressure must have been above the triple point pressure (0.46 GPa). Other one-component mineral polymorphs such as calcite and aragonite or quartz and coesite offer T-P constraints, though with only two phases and one reaction curve there is no invariant point at normal metamorphic conditions.

Als Phase Diagram

Figure 6.04. T or P Limiting Assemblages. Intersecting reaction curves define fixed points that can be used to place limits on temperature (T) and pressure (P) for assemblages of minerals involved in the reactions. Click on the image to see a larger version with more information.

Mineral assemblages and reactions involving two or more components also offer T-P constraints if reaction curves intersect. For example, the "staurolite out" (St = Grt + Bt + Als) reaction curve intersects two of the Als reaction curves as shown in Figure 6.04. These intersections define temperature and pressure maxima and minima that apply to equilibrium assemblages of two minerals. The assemblage Bt-Ky can occur at equilibrum only at pressures above 0.77 GPa. Similarly, the assemblage Bt-Sil can occur at equilibrum only at temperatures above 608°C. These minerals occur in metamorposed shales. Similar fixed point constraints occur in other bulk compositions. For example, the assemblage enstatite + talc in a metamorphosed ultramafic rock requires a minimum presure of 0.64 GPa (see this T-P diagram). You can read more about conditions limits like these in Carmichael (1978).

Bounds to the T-P conditions of metamorphism are helpful, but perhaps not as informative as actual temperatures or pressures. To this end, petrologists have calibrated geothermometers and geobarometers based on the chemical compositions of certain solid solution minerals.