An empirical model for the calculation of spinel-melt equilibria in mafic igneous systems at atmospheric pressure: 2. Fe-Ti oxides

In order to develop models simulating the crystallization of Fe-Ti oxides in natural lavas, we have processed published experimental data on magnetite-melt and ilmenite-melt equilibria. These data include 62 Mt-melt and 75 Ilm-melt pairs at temperatures 1040-1150 degrees C, oxygen fugacities from IW to NNO + 2, and bulk compositions ranging from ferrobasalts to andesites and dacites. Five major cations (Fe3+, Fe2+, Ti4+ Mg2+ and Al3+) were considered for the purpose of describing Fe-Ti oxide saturation as a function of melt composition, temperature and oxygen fugacity at 1 atmosphere pressure. The empirically calibrated mineral-melt expression based on multiple linear regressions is: ln D-i = a/T + blog f(O2) + c + d(1)X(Na) + d(2)X(K) + d(3)X(P), where D-i represents molar distribution coefficients of the given cations between Mt/Ilm and melt; X-Na, X-K, and X-p are the molar fractions of Na, K, and P in the melt. The empirically calibrated Mt-melt and Ilm-melt equilibria equations allowed us to develop two models for calculating crystallization temperatures of the Fe-Ti oxides in the melts with an accuracy of 10-15 degrees C, and compositions with an accuracy of 0.5-2 mol%. These models have been integrated into the COMAGMAT-3.5 program, improving our ability to study numerically the effects of temperature and oxygen fugacity on the stability and phase equilibria of Fe-Ti oxides. Application of this approach to the tholeiitic series of Chazhma Sill from Eastern Kamchatka (Russia) indicates oxygen fugacity conditions near NNO + 0.5. Numerical simulation of fractional crystallization of an iron-enriched basaltic andesite parent at these oxidizing conditions accurately reproduces the FeO-SiO2 relations observed in the Chazhma suite.