The effects of temperature, oxygen fugacity and melt composition on the behaviour of chromium in basic and ultrabasic melts

Abstract

Chromite was equilibrated with two natural basic liquids and one natural ultrabasic liquid at temperatures and oxygen fugacities appropriate to geological conditions. The experiments were designed to document changes in mineral and glass compositions between the iron-wüstite and nickel-nickel oxide buffers, with special emphasis on conditions along quartz-fayalite-magnetite. The Cr contents of the melts at chromite saturation increase strongly with increasing temperature and with decreasing oxygen fugacity. A relationship is described which accounts for the compositional dependence of the partitioning of Cr between spinels and silicate melts by considering the exchange of FeCr 2O 4 component between the crystalline and melt phases. Interpretation of the data in terms of this exchange suggests that Cr 3+ in metaluminous melts occurs in octahedrally coordinated sites, and that it does not depend on charge-balancing by monovalent cations. In this model, Cr 3+ is proposed to behave like network-modifying Al 3+ and Fe 3+, i.e., the excess aluminum and ferric iron which do not participate in tetrahedrally coordinated matrix or network-forming complexes. The results can also be applied to the problem of the formation of massive chromitites of great lateral extent in basic layered intrusions. The data are consistent with a model in which the crystallization of chromite is initiated through magma mixing, in combination with the rapid heat loss associated with periodic influxes of magma into a chamber. An alternative model, in which chromite crystallization is initiated by repeated fluctuations in oxygen fugacity, is possible only if the magma f O 2 is not controlled by an oxygen buffer such as QFM.