CHEMICAL MASS-TRANSFER IN MAGMATIC PROCESSES .4. A REVISED AND INTERNALLY CONSISTENT THERMODYNAMIC MODEL FOR THE INTERPOLATION AND EXTRAPOLATION OF LIQUID-SOLID EQUILIBRIA IN MAGMATIC SYSTEMS AT ELEVATED-TEMPERATURES AND PRESSURES

A revised regular solution-type thermodynamic model for twelve-compone nt silicate liquids in the system O2-Al2O3-Fe2O3-Cr2O3-FeO-MgO-CaO-Na2 O-K2O-P2O5-H2O is calibrated. The model is referenced to previously pu blished standard state thermodynamic properties and is derived from a set of internally consistent thermodynamic models for solid solutions of the igneous rock forming minerals, including: (Mg, Fe2+ Ca)-olivine s, (Na, Mg, Fe2+, Ca)(M2) (Mg, Fe2+, Ti, Fe3+, Al)(M1) (Fe3+, Al, Si)( 2) O-TET(6-) pyroxenes, (Na,Ca,K)-feldspars, (Mg, Fe2+) (Fe3+, Al, Cr) (2)O-4-(Mg, Fe2+)(2) TiO4 spinels and (Fe2+, Mg, Mn2+)TiO3-Fe2O3 rhomb ohedral oxides. The calibration utilizes over 2,500 experimentally det ermined compositions of silicate liquids coexisting at known temperatu res, pressures and oxygen fugacities with apatite +/- feldspar +/- leu cite +/- olivine +/- pyroxene +/- quartz +/- rhombohedral oxides +/- s pinel +/- whitlockite ) water. The model is applicable to natural magm atic compositions (both hydrous and anhydrous), ranging from potash an karatrites to rhyolites, over the temperature(T) range 900 degrees-170 0 degrees C and pressures (P) up to 4 GPa. The model is implemented as a software package (MELTS) which may be used to simulate igneous proc esses such as (1) equilibrium or fractional crystallization, (2) isoth ermal, isenthalpic or isochoric assimilation, and (3) degassing of vol atiles. Phase equilibria are predicted using the MELTS package by spec ifying bulk composition of the system and either (1) T and P, (2) enth alpy (H) and P, (3) entropy (S) and P, or (4) T and volume (V). Phase relations in systems open to oxygen are determined by directly specify ing the f(O2) or the T-P-f(O2) (or equivalently H-P-f(O2), S-P-f(O2), T-V-f(O2)) evolution path. Calculations are performed by constrained m inimization of the appropriate thermodynamic potential. Compositions a nd proportions of solids and liquids in the equilibrium assemblage are computed.