CRYSTALLIZATION OF CHROMITE AND CHROMIUM SOLUBILITY IN BASALTIC MELTS

Abstract

The equilibrium between chromite and melt has been determined on four basalts at temperatures of 1200–1400°C over a range of oxygen fugacity (fo2) and pressures of 1 atm and 10 kb. The Cr content of chromite-saturated melts at 1300°C and 1 atm ranges from 0?05 wt.% Cr2O3 at a log fo2= ?3 to 1?4 wt.% at a log fo2=?12?8. The Cr2+/Cr3+ of melt increases with decreasing fo2 and is estimated by assuming a constant partitioning of Cr3+ between chromite and melt at constant temperature. The estimated values of Cr2+/Cr3+ in the melt are at fo2 values of 4–5 orders of magnitude lower than the equivalent Fe2+/Fe3+ values. The Cr/(Cr+Al) of chromite coexisting with melt at constant temperature changes little with variation of fo2 below log fo2=?6. Five experiments at 10 kb indicate that Cr2O3 dissolved in the melt is slightly higher and the Cr/(Cr + Al) of coexisting chromite is slightly lower than experiments at 1 atm pressure. Thus variation in total pressure cannot explain the large variations of Cr/(Cr + Al) that are common to mid-ocean ridge basalt (MORB) chromite. Experiments on a MORB at 1 atm at fo2 values close to fayalite-magnetite-quartz (FMQ) buffer showed that the Al2O3 content of melt is highly sensitive to the crystallization or melting of plagioclase, and consequently coexisting chromite shows a large change in Cr/(Cr + Al). It would appear, therefore, that mixing of a MORB magma containing plagioclase with a hotter MORB magma undersaturated in plagioclase may give rise to the large range of Cr/(Cr + Al) observed in some MORB chromite.