The partial molar volume of Fe₂O₃ in multicomponent silicate liquids and the pressure dependence of oxygen fugacity in magmas

Abstract

Abstract Density measurements of eight silicate liquids containing substantial amounts of Fe 2 O 3 have been made over a range of 250 °C. These have been combined with published density measurements on multicomponent silicate liquids to yield (by multiple regression) partial molar volumes of SiO 2 , TiO 2 , Al 2 O 3 , Fe 2 O 3 , FeO, MgO, CaO, Na 2 O, and K 2 O. The data on Fe 2 O 3 -liquids are neither precise nor abundant enough to show a compositional dependence of . In a liquid of constant composition and temperature, the pressure dependence of the oxygen fugacity is given by which, if Δ V is independent of pressure, necessitates an increase in f O 2 with increasing pressure of about 1 log 10 unit for 10 kbars. Combining an equation relating oxygen fugacity to composition, T , and Fe 2 O 3 at 1 bar (Sack et al. , 1980) with the results for partial molar volumes, the oxygen fugacity of any magma can be calculated as a function of P and T. If basic magmas have their Fe 2 O 3 /FeO set in the source regions, and ascend isochemically, then the calculated oxygen fugacities in the mantle increase as pressure increases and silica activity decreases. A P-T grid has been constructed to show the calculated oxygen fugacities in a source region which has equilibrated with some common lava types, based on their FeO and Fe 2 O 3 contents.