THERMODYNAMICS OF MULTICOMPONENT PYROXENES .3. CALIBRATION OF FE2-1),TIAL2(MGSI2)(-1), TIFE2(3+)(MGSI2)(-1), ALFE3+(MGSI)(-1), NAAL(CAMG)(-1), AL-2(MGSI)(-1) AND CA(MG)(-1) EXCHANGE-REACTIONS BETWEEN PYROXENES AND SILICATE MELTS((MG)()

A thermodynamic model for the Gibbs free energy of igneous pyroxenes w ith the general formula [Na, Ca, Fe2+, Mg](M2)[Fe2+, Mg, Ti, Al, Fe3+] (M1)[Al, Fe3+, Si](SiO6)-Si-Tet is calibrated from experimentally dete rmined compositions of coexisting pyroxene and silicate melt. The mode l is based upon the general formulation, and relies upon the calibrati on of the ''quadrilateral'' subsystem, previously published by the pre sent authors. The calibration database of pyroxene-hquid equilibria sp ans a broad spectrum of temperature, pressure and oxygen fugacity cond itions, ranging from 1000 degrees-1600 degrees C, 0.001-30 kbar and ir on-wustite to air. Chemical potentials of endmember pyroxene component s as well as exchange potentials between pyroxenes and coexisting liqu ids are defined utilizing the present authors thermodynamic melt model . Model parameters are extracted from these relations by regression an alysis. The resulting model and derivative endmember properties are in ternally consistent with an existing standard state thermodynamic data base. The success of the model and its applicability to igneous petrog enesis are demonstrated by comparing calculated and experimentally det ermined liquidus compositions, temperatures and symmetry states for py roxenes crystallizing from a variety of silicate melts, ranging in com position from tholeiites and angrites through rhyolites to potash anka ratrites.