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

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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 $$log f_{O_{2}}$$ diagrams covering the $$log f_{O_{2}}$$ 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 $$10^{-8} atm$$ 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 $$log f_{O_{2}}$$ 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.

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