The Crystallization of Spinel from Basaltic Liquid as a Function of Oxygen Fugacity

Abstract

The effect of changing oxygen fugacity on the equilibrium crystallization of two basaltic melts has been determined at 1 atm total pressure. Liquidus curves for spinel (chromite-titaniferous magnetite), hexagonal oxide, olivine, pyroxene, and plagioclase are represented on temperature versus diagrams covering the range -0.68 (air) to -14.0 atm and a temperature range of l,100°C-1,325°C. Particular emphasis has been placed on the role of oxygen fugacity as applied to the relationship between chromite and titaniferous magnetite and the ferrous-ferric ratio. Complete solid solution between chromite and titaniferous magnetite is demonstrated at basaltic liquidus temperatures, however at oxygen fugacities below about the crystallization of early chromite is interrupted by the crystallization of clinopyroxene. At lower temperatures, a spinel again crystallizes as titaniferous magnetite together with pyroxene and other silicates. This supports the suggestion of Irvine that a reaction involving chromite, silicate liquid, and pyroxene is responsible for the hiatus in crystallization often found in layered basic intrusions between early chromite and a later titaniferous magnetite. The maximum solubility of chromium in basaltic liquids at l,200°C and a of - 8 is approximately 200 ppm. The very low solubility of chromium in basaltic liquid is in part responsible for the very high enrichment of chromium in chromite relative to basaltic liquid (1,000-fold) and for the small size of chromite crystals relative to the other early magmatic minerals.