AMPHIBOLE-LIQUID EQUILIBRIA: BAROMETERS AND THERMOMETERS FOR VOLCANIC SYSTEMS

Geobarometers for igneous systems have been calibrated using Al in amphibole, allowing P to be estimated independent of knowledge of an equilibrium liquid composition. Such barometers, however, require that Al activity is fixed by co-existing phases, and so are not applicable to volcanic systems. An exception is Ridolfi and Renzulli (2012), who use multiple amphibole components as proxies for liquid compositions; their Eqn. 1d appears to be uniquely effective in reproducing experimental P. As an alternative, a new model is calibrated based on Al partitioning between amphibole and liquid, D(Al) = X_Al^amph/X_Al2O3^liq:

P(GPa) = -3.093-4.274[lnD(Al)] - 4.216[ln(X_Al^liq)] + 63.3[X_P2O5^liq] + 1.264[X_H2O^liq]

+ 2.457[Al^amph] + 1.86[K^amph] + 0.4[ln(Na^amph / X_Na2O^liq)].

Here, liquid components are hydrous mole fractions of the oxides, and amphibole components are numbers of cations on the basis of 23 oxygens; R² = 0.89; SEE = ± 0.15 GPa; n= 157; P range = 0.05 – 2.5 GPa; compositional range = 34-76% SiO₂; 0.1-10% MgO; 2.5-11% TA. New experiments are needed to test this expression and Eqn. 1d of R&R, but the new equation predicts mean P for select experimental data (not used in either calibration) with higher precision: Sisson et al. (2005; 0.7 GPa; P(calc)=0.71±2.2 GPa; n=49), Krawczynski et al. (2012; 0.5 GPa; P(calc) = 0.46 ±0.1 GPa; n=10) and Feig et al. (2010) and Wolf et al. (2012) (0.2 GPa; P(calc) = 0.26±0.13 GPa; n=5). Temperatures of amphibole saturation can be predicted using: 10⁴/T(K) = 6.5677-0.2163[P(GPa)] -3.6673[X_SiO2^liq]+3.0595[ln(X_SiO2^liq)] - 1.561[ln(X_Al2O3^liq)] - 0.13532[ln(X_Na2O^liqX_Al2O3^liq)] -0.19167[ln((X_MgO^liq+X_FeOt^liq)(X_Al2O3^liq))], where X_FeOt is the hydrous mole fraction of Fe, when calculated from weight % values expressed as FeO total (R² = 0.83; SEE = ± 33 ^oC; n= 146; T range = 678-1130^oC). Also useful, to predict saturated water contents in liquids is: H₂O(wt. %) =0.7996+15.347[P(GPa)]^0.5 - 0.00233[T(^oC)] + 0.06248[Na₂O+K₂O wt. %)], where R² = 0.85, SEE = ±1.07 wt. % H₂O, n=1,122. And as a check on equilibrium: [(X_FeOt/X_MgO)^amph]/[(X_FeOt/X_MgO)^liq] = 0.28±.0.11 (n=449); but Al does not equilibrate as readily as Fe-Mg. The new models are expected to be generally applicable to igneous systems and should allow increased precision of intensive parameters for volcanic systems.