6.10 Summary

  • The aluminosilicate (Als) minerals andalusite, kyanite, and sillimanite 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.
  • Mineral assemblages and reactions involving two or more components also offer T-P constraints if reaction curves intersect. These intersections define temperature and pressure maxima and minima that apply to equilibrium assemblages of two minerals.
  • When in equilibrium with other phases, the chemical compositions of solid solution minerals can depend directly on temperature. Therefore, the compositions of solid solution minerals have the potential be used as geothermometers.
  • The Fe/Mg values of garnet and biotite in equilibrium have been calibrated as a geothermometer. Unfortunately, possible changes in the mineral Fe/Mg values by diffusion as the rock cools, particularly for biotite, increase the uncertainty of the temperatures of the Grt-Bt geothermometer.
  • Geobarometers have been calibrated from chemical reactions that have a significant change in volume, which leads to a lower slope (less vertical) on a T-P diagram. Most geobarometers also depend on the chemical compositions of solid solution minerals in the reactions.
  • If a metamorphic rock has the minerals for both a geothermometer and a geobarometer, it may be possible to determine both the temperature and pressure at which the minerals equilibrated.
  • Thermodynamic models for the solid solution properties of the minerals used in geothermometers and geobarometers are needed to get good results when the mineral compositions in a metamorphic rock are not the same as the mineral compositions in the laboratory calibration experiments.
  • If thermodynamic models are available for all of the minerals in a rock, a petrogenetic grid can be calculated that shows chemical reactions that may occur as a function of temperature and pressure. Petrologists have developed such models and continue to refine them to be consistent with laboratory experiments and mineral assemblages observed in metamorphic rocks.
  • If a specific whole rock composition is specified, the thermodynamic models can be used to calculate a mineral assemblage diagram (MAD, also called pseudosection) that shows all of the discontinuous reactions that can occur at equilibrium for that rock.
  • Petrologists use a variety of information (e.g. mineral assemblages, mineral compositions, rock textures, reaction textures, mineral inclusion, and thermodynamic models) to constrain the pressure-temperature (P-T) path that a metamorphic rock has followed from the formation of its protolith through burial and heating to metamorphic conditions to eventual return to the surface by erosion, tectonic movement, or both.
  • P-T paths can become pressure-temperature-time (P-T-t) paths if age data are determined for minerals in the metamorphic rocks, providing important constraints on the way the earth works to create metamorphic rocks.