6.5 GASP Geobarometer

The best geobarometers use chemical reactions that have a significant change in volume, which leads to a lower slope (less vertical) on a T-P diagram. The equilibrium conditions of one such reaction

grossular (Grs) + kyanite (Ky) + quartz (Qz) = anorthite (An) (4)

are shown in Figure 6.10 as a green line based on the experiments of Andrea Koziol and Robert Newton (1988). Although these end-member mineral compositions are not likely to occur in common metamorphic rock bulk compositions, the reaction can be corrected for the Ca-contents of both garnet and plagioclase. Using thermodynamic data for the minerals in the reaction, the Koziol and Newton (1988) experimental results can be expressed as:

-48357 + 150.66*T - 6.608*(P-1) + R*T*ln(K_eq) = 0 (5)

(equation 15-48 of Spear (1993)) where T is temperature (K), P is pressure (bars), R is the gas constant (8.3145 J/K), and K_eq is an equilibrium constant that can be defined in terms of the garnet and plagioclase compositions. Assuming a simple ionic solution model:
K_eq = [(X_Grs)^Grt/(X_An)^Pl]³ (6)

where (X_Grs)^Grt is the mole fraction of the grossularite component in garnet and (X_An)^Pl is the mole fraction of the anorthite component in plagioclase. Figure 6.11 shows both the GASP barometer and the Grt-Bt thermometer on the same T-P diagram. Click on the diagram to see a larger version with input boxes to enter compositional information for Grt and Bt, and for Grt and Pl. The temperature and pressure at which the red thermometer line and the green barometer line intersect identify the equilibrium conditions for the assemblage Grt-Bt-Pl-Ky-Qz with mineral compositions of Grt, Bt, and Pl that match the (K_D)^Grt-Bt and the (K_eq)^GASP.

Combining a geothermometer with a geobarometer yields a temperature and pressure. This seems like a great result, but there are problems and uncertainties. Some of these are considered on the next pages.

Solid Solution Issues