EMPIRICAL SOLUTION MODEL FOR ALKALIC TO THOLEIITIC BASIC MAGMAS

Currently available models to simulate naturally occurring mineral-mel t equilibria use mineral components limited to tholeiitic basalt compo sitions and thus they cannot be used for alkali-rich basalts and basan ites. To expand mineral-melt equilibria calculations to alkali-rich co mposition space at low pressures, we have derived equations that descr ibe chemical equilibria between olivine-melt, pyroxene-melt, plagiocla se-melt, nepheline-melt and leucite-melt components. Excess free energ ies of reactions between the end-member mineral and melt components at equilibrium have been expressed as a function of melt composition, te mperature and f(O2). The database used to calculate the mineral-melt e xpressions consists of a total of >350 anhydrous experiments conducted under controlled oxygen fugacity defined by the quartz-fayalite-magne tite (QFM) oxygen buffer. Rocks used in these experiments range from b asanites, nephelinites and alkali olivine basalts, to tholeiitic basal ts and basaltic andesites. Using bulk compositions of starting materia ls both in this experimental database and in others that were not inco rporated into the regression of modeled parameters, modeled equations successfully predict, at a given temperature and fo, compositions of m ultiply saturated melts as well as the compositions of coexisting mine rals. Standard deviations of the calculated male fractions of mineral components (sigma) are as follows: anorthite 0.02; forsterite 0.02; cl inoenstatite 0.02; enstatite 0.003; nepheline 0.02; and leucite 0.01. Standard deviations (sigma) of the calculated melt compositions in ter ms of weight percent of oxides are: SiO2 0.96; Al2O3 1.32; Fe2O3 0.23; FeO 1.21; MgO 0.84; CaO 0.79; Na2O 0.58; and K2O 0.69. All calculatio ns were carried out using a non-linear Newton-Raphson numerical proced ure.