ALBERT

Notes: Abstract Low pressure phase equilibria for natural mafic systems may be calculated utilizing empirically derived single component distribution coefficients and the constraints placed upon mineral-melt equilibria by stoichiometry and mass balance, without any knowledge of thermochemical properties of melts (Langmuir and Hanson 1981). Variations in distribution coefficients caused by differences in melt composition may be largely eliminated by the application of a two-lattice melt model based upon the models of Nielsen and Drake (1979), Drake (1976b), and Bottinga and Weill (1972). In the two-lattice melt model, the melt is assumed to be made up of two independent quasi-lattices, the network formers, composed of the components SiO2, NaAlO2, and KAlO2, and the network modifiers, composed of the free oxides of Ca, Mg, Fe, Al, Ti, and Cr. Compositionally independent mineral-melt distribution coefficients were calculated for all the major and some minor components in olivine, plagioclase, high-Ca pyroxene, low-Ca pyroxene, spinel, and ilmenite. Regression constants were calculated from data from 1 atmosphere, anhydrous, equilibrium experiments on natural and synthetic mafic compositions including data obtained from the literature and from new synthetic spinel-melt experiments in the Fo-Ab-An system doped with Ti, Mn, Ni, and Fe. The distribution coefficients are internally consistent within 3 mole % (1σ) for all components. The effects of variable oxygen fugacity on mafic mineral-melt equilibria were calculated utilizing the relations of Sack et al. (1980) for the determination of the Fe+3/Fe+2 ratio, and the results of Schreiber and Haskin (1976) for Cr+3/Cr+2. A computer program, EQUIL.FOR, incorporating the derived mineral-melt distribution coefficients, calculates the sequence of mineral and melt compositions, and liquidus temperatures for mafic melt compositions undergoing equilibrium or fractional crystallization. This program reproduces, within 15° C, 52 experimentally determined crystallization sequences collected from the literature. These published crystallization sequences are for mafic systems not represented in the data set used to calculate the mineral-melt distribution coefficients.