CALCULATION OF TITANOMAGNETITE STABILITY ON THE LIQUIDUS OF BASALTS AND ANDESITES WITH SPECIAL REFERENCE TO THOLEIITIC MAGMA DIFFERENTIATION

Abstract

A detailed review of experimental data on magnetite-melt equilibria in systems ranging from basalts to dacites is presented. This information (99 experiments) was processed statistically to yield the fol- lowing equation relating the crystallization temperature of magnetite to melt composition and oxygen fugacity: For 90% of the data, this equation reproduces experimental temperatures to a precision of ∠ 15°C ; the average deviation of the calculated temperatures from experimental ones is 9.3°C. The crystallization temperature of magnetite shows a complex dependence on oxygen fugacity: for ferrobasaltic and andesitic melts with moder- ate contents of Fe (up to ~ 15 wt %) and TiO 2 (up to ~ 2 wt %), there is a fairly steep slope of the magnetite liquidus ~ 20¡C/ , whereas, for high-Ti (4-5 wt % TiO 2 ) and Fe-rich (17-18 wt %) basalts, liquidus temperatures are virtually independent of oxygen fugacity and are near 1100°C. The equation for the magnetite liquidus can be used as an empirical basis for improving computer models of magma crystallization involving the separation of magnetite-bearing mineral assemblages. 10 4 T ∨ b 0 b 1 X SiO2 b 2 X TiO2 b 3 X FeOtot +++ () f O2 cd 1 X SiO2 d 2 X TiO2 d 3 X FeOtot d 4 X P2O5 . ++ + + + log =